(CNN) The dead submarine crew hadn't moved from their stations for nearly 150 years when the vessel was raised from the ocean in 2000. Whatever killed them happened so suddenly that they never made a run for the escape hatch. What's more, they had no obvious physical injuries. 
 
 There was no major damage to the hull that could be definitively traced back to the day the H.L. Hunley, a 40-foot-long Confederate submarine, sank to the ocean floor off Charleston, South Carolina, on February 17, 1864. 
 
 Researchers had unsealed the crew compartment of the submarine, but they have yet to find conclusive evidence of how the eight men aboard died. 
 
 But in research published Wednesday in the journal Plos One one group of scientists thinks they've finally cracked the case of what killed the crew so swiftly. 
 
 Shrinking down, blowing up 
 
 The Hunley became the first sub to sink an enemy ship in battle: the USS Housatonic. But sometime after, it went down, too. 
 
 It sank the enemy ship with a 135-pound torpedo, which was filled with black powder and attached to a pole 16 feet from the ship's hull. The study authors say the torpedo is the key -- but many have wondered how an explosion could've killed the entire crew without leaving a trace. 
 
 Rachel Lance set off explosions near a miniature replica of a Civil War submarine. 
 
 To answer this question, biomechanist Rachel Lance designed a model of the Hunley, one-sixth the length of the 40-foot-long submarine. The model, built by Durham-based sculptor Tripp Jarvis, was christened the CSS Tiny. 
 
 Lance, then a graduate student at Duke University and an engineer with the Naval Surface Warfare Center , decided she would set off test explosions next to the model submarine. So she found an eight-acre pond on a family-run farm in St. Louis, North Carolina. Bert Pitt of Pitt Family Farms agreed to let Lance use the pond to conduct her experiments. 
 
 "Initially, when she was talking about blasting, I was a little concerned," said Pitt, 65, a sixth-generation family farmer, whose grandchildren now make eight generations. 
 
 Pitt recalled the wires snaking into the lake and the charges that detonated beneath the surface, splashing water into the air like a large firecracker, he said. One of his grandkids got to press the button. 
 
 "It had a little geyser to it," he said. "It was neat to see." 
 
 Pitt, a self-proclaimed history buff, had always been interested in the Civil War. He has ancestors who were in the North Carolina Regiments, and at least one of them is buried in their own family graveyard. The house he lives in was built in 1830, before the Hunley sank. 
 
 He keenly eyed reports about the Hunley on the History Channel and the National Geographic Channel. 
 
 "They were sitting perfectly still in that submarine," Pitt said. "I think people would like to know what did happen to the crew. Everything about the story is intriguing." 
 
 Without a trace 
 
 Suspended inside the CSS Tiny was a small pressure gauge, which revealed how the sub's own torpedo blast could have killed the Hunley crew without leaving a lasting mark: the shock wave created by the blast. 
 
 The shock wave hit the Hunley's hull, which was less than an inch thick, said Lance, lead author of the new study. The metal bent ever so slightly but fast enough to transfer the blast wave to the inside of the cabin. 
 
 That wave then traveled through the cabin, hitting each of the eight crewmembers, traveling through their bodies. But the real damage, Lance said, probably occurred when the pressure wave reached their lungs. 
 
 "The issue is when it's passing through (the tissues) and it suddenly hits air," she said. 
 
 Shock waves, like sound waves, travel quickly in water and solids but not air. The wave slows as it hits the lung, Lance said, and "that energy has to transmit somewhere." 
 
 The end result: The blood vessels in the lungs can rupture, known as a pulmonary hemorrhage. 
 
 "It was ... noted that men could be killed or disabled at considerable distance" from an explosive, Dr. Thomas Chiffelle, a pathologist from Albuquerque, New Mexico, wrote in a 1966 report for the US Department of Defense. "The man or animal may be killed outright, without external signs of injury, but often with blood-tinged froth or frank blood appearing in the nose and mouth." 
 
 It is possible to survive a blast wave from far enough, according to Chiffelle's accounts. Witness accounts from the night of the Hunley's sinking claimed that there was a blue light coming from the ocean. Some speculated that it was the Hunley crew signaling that they'd accomplished their mission. 
 
 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Since 2000, scientists, historians and a genealogist have studied the first submarine to sink an enemy vessel. The H.L. Hunley did just that more than 150 years ago, on February 17, 1864, during the American Civil War. Hide Caption 1 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – The 40-foot submarine was brought up amid much fanfare off Charleston, South Carolina, in August 2000. Author Clive Cussler and a team discovered the Hunley five years earlier, buried in the sand more than 100 yards beyond its target, the USS Housatonic. Hide Caption 2 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Conservators later began a new process -- filling the tank that holds the Hunley with chemicals that helped strip away what is called "concretion": organic material that has coated the hull and interior. Hide Caption 3 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Visitors to a laboratory and exhibit hall in North Charleston, South Carolina, can gaze down on the Hunley on weekends. The large tank is empty when scientists in the Hunley Project are at work. Hide Caption 4 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – The hull is in pretty good shape, despite exposure to sea currents and elements for decades. The eight-member Confederate crew, sitting on the port side, turned a propeller by hand. Hide Caption 5 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Several years ago, then-senior archaeologist Maria Jacobsen found a gold coin that belonged to submarine commander Lt. George Dixon. Hide Caption 6 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Lt. George Dixon, according to legend, received this coin as a good-luck charm from his beloved, said to be from Mobile, Alabama. It was bent by a bullet when he was wounded at the Battle of Shiloh, two years before the Hunley made history. He had it engraved and carried it the rest of his life. It was found with his remains by Hunley Project scientists. Hide Caption 7 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Paul Mardikian, left, senior conservator, and Philippe de Vivies remove material from a piece of the submarine. Hide Caption 8 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – One of the personal belongings found inside the Hunley, a watch belonging to Lt. George Dixon. Hide Caption 9 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – A wallet found in the shipwreck helps tells the story of ordinary life onshore. Hide Caption 10 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Crew member James Wicks' bandana after conservation. Hide Caption 11 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – This burnt matchstick was found buried in the sediment-filled interior of the H.L. Hunley. Hide Caption 12 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – Experts believe this lantern was used as a flashlight by the submarine's commander. Hide Caption 13 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – This conserved oil can almost looks good as new. Hide Caption 14 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Confederate submarine H. L. Hunley – The bowl of a pipe belonging to Confederate sailor Joseph Ridgaway, the only crew member positively identified through DNA. Hide Caption 15 of 16 Photos: Delving into mysteries of submarine H.L. Hunley Delving into mysteries of submarine H.L. Hunley – Experts examine a shoe found in the H.L. Hunley. Hide Caption 16 of 16 
 
 But Lance, who is working on a book about the Hunley, said that she has doubts about inconsistencies in these testimonies. 
 
 It is virtually impossible to know how powerful the Hunley's torpedo blast was, even with the amount of black powder used. The blast can also change with how tightly the powder is packed and how fine the grains are, according to the Naval History and Heritage Command. Replicating the black powder explosion, Lance said, was the trickiest part of the experiment. 
 
 So Lance lowballed it, testing several blasts in the process. She concluded that the shock wave would have instantly killed those aboard the Hunley, based on her calculations and a wealth of prior air blast experiments on large animals. 
 
 "Any explosive we've seen in the field ... would definitely create a lethal wave," Lance said. 
 
 "These types of injuries are not subtle," she added. "The damage is immediate." 
 
 Tick tock 
 
 There was another piece of evidence that stood in her favor: a gold pocket watch that belonged to the Hunley's captain, Lt. George Dixon. 
 
 The watch had stopped at 8:23, about the time of the Hunley's attack, historians believe. 
 
 "Most importantly, it appears it didn't wind down naturally," according to a 2007 update by a research partnership known as the Hunley Project. "Something traumatic -- perhaps water, a shock wave, or some other intervening force -- caused it to stop at that precise time." 
 
 Friends of the Hunley -- part of the Hunley Project, which was not involved in the new research -- declined to comment on the research. The organization maintains and researches the original submarine. 
 
 Prior naval research has concluded that "neither phase of the explosion was severe enough ... to have significantly impacted Hunley." 
 
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 "We had a lot of submariners survive being depth-charged at very close quarters during WWII," said Paul Taylor, a spokesman at Naval History and Heritage Command. "You sort of wonder how they did OK, but supposedly the folks in the Hunley didn't." 
 
 The Navy researchers who have been examining the Hunley for over a decade declined to comment on Lance's study while their own research on the crew deaths is ongoing. 
 
 But Lance, for one, said she feels like this part of the mystery has been solved. 
 
 "This project was originally intended to be a side project, and then it spiraled out of control when we realized we could do actually do it," she said. ||||| During the latter stages of the American Civil War, the H.L. Hunley made history by becoming the first combat submarine to sink an enemy ship. The Confederate crew never returned from its mission, sparking a mystery that’s lasted for over 130 years. An exhaustive new analysis suggests these pioneering submariners didn’t drown or suffocate as commonly believed, but instead died from the shockwave triggered by their very own weapon. 
 
 The eight-man crew of the 40-foot Confederate submarine H.L. Hunley died instantly when its powerful barrel torpedo exploded into the hull of the 1,200-ton Union warship USS Housatonic, according to new research published today in PLoS One. Duke University biomechanist Rachel Lance’s exhaustive, three-year investigation suggests the explosion caused minimal damage to the sub, which was less than 20 feet away, but the ensuing shockwave caused catastrophic injuries to the crewmen’s soft tissues, especially to their brains and lungs. The finding subsequently fills in an important gap in US Civil War history, while offering fresh insights into human physiology and the physical forces we’re able—or unable—to endure. 
 
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 On the evening of February 17, 1864, the H.L. Hunley, commanded by Charles Pickering, embarked on what would be its first and final combat mission. As part of the Confederate campaign to dismantle the stifling Unionist naval blockade, the sub’s crew managed to sink the USS Housatonic with a submersible barrel bomb. 
 
 This “torpedo” wasn’t a self-propelled bomb in the conventional sense. Rather, it was a copper keg filled with 135 pounds of black powder that was positioned slightly ahead and below the sub’s bow at the tip of a 16-foot pole known as a spar. The crew plunged this bomb into the Housatonic’s hull just below the waterline, causing a tremendous explosion. The Union warship sunk in just five minutes, killing five of her crew. The ship came to rest upright in 30 feet of water, allowing the remaining seamen to be rescued. 
 
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 The mission may have been a success, but the crew of H.L. Hunley was never heard from again. Its fate was finally revealed in 1995 when the sunken sub was found a mere 300 meters from the wreck of the Housatonic near Charleston Harbor in South Carolina. The sub was raised in 2000, and it has been under investigation by Clemson University scientists ever since. 
 
 
 
 But while the sub’s fate was finally revealed, it still wasn’t clear what transpired during the military engagement, and what led to the death of its eight seamen. Historians have speculated that the Hunley crew drowned or suffocated, or that shear forces broke a valve causing the sub to sink—or even that the H.L. Hunley was shot by a seaman aboard the doomed Housatonic. 
 
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 But none of these explanations fit the evidence. Confusingly, the crewmen’s skeletons were all found at their hand-crank stations, which were used to manually propel the craft. They exhibited no broken bones, the drain pumps were closed, as were the air hatches. The sub was startlingly intact, the only noticeable blemishes being a hole in one conning tower and an apparently broken window. The only satisfactory explanation, argues Lance, is that the crew was killed instantly by the shockwave generated from the barrel bomb explosion. 
 
 To prove her case, Lance constructed a 6-1/2-foot scale model of the sub and subjected it to pressurized air blasts and black powder explosions while monitoring the effects with a series of onboard sensors. With the help of a Civil War reenactor and his fully functional, historically accurate rifle, she shot at period-accurate iron plating meant to mimic the sub’s hull. She analyzed the effects of energy blasts onto the human respiratory system, pored through documents at the National Archives in Washington, interviewed an ATF explosives expert, and even visited an original black powder mill. The whole endeavour took about three years, but the results are compelling. 
 
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 Lance’s conclusion is that all crew members died instantly from the tremendous force of the explosion. The ensuing shockwave travelled through the soft tissues of seamen’s bodies, especially their lungs and brains. 
 
 Normally, a shockwave of this strength would travel at a rate of 340 meters per second (m/s) through the open air, but underwater it accelerates to 1,500 m/s. By the time this shockwave hit the sub and penetrated the crewmen’s skin (the distance to the explosion varied for each crew member, ranging from 18 to 42 feet depending on where they were positioned in the sub), it slowed down to 30 m/s, which was still enough to cause catastrophic physical damage. Each seaman was subjected to about 60 milliseconds of shockwave trauma, as compared to about 10 milliseconds had the men been exposed above water. Lance assesses the probably of this happening to each crew member at about 85 percent. 
 
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 “Injuries and fatalities from blasts occur instantaneously,” Lance told Gizmodo. “Since we calculated the blast exposure levels to be in the lethal range, the crew probably did not even have time to realize what occurred. They would have known they were approaching the Housatonic and preparing to attack, but once the torpedo exploded they would have suffered fatal pulmonary and brain traumas before they processed that their attack had been successful.” 
 
 
 
 This cause of death would have left no mark on the skeletal remains. The soft tissues, which would have shown what happened, are long gone. But in addition to this shockwave analysis, other evidence exists to show that the men died instantly. Had they survived the blast, the crewmembers would’ve tried to release the keel ballast weights, start pumping water, or make an effort to escape the sub—but it appears none of these actions were taken. 
 
 
 
 Lance says these findings hold dual significance. 
 
 “As an injury biomechanist it is a fascinating example of a completely unprecedented injury mechanism,” she said. “The physiology of human beings has not really changed in the past 150 years with the exception of better nutrition, so humans in 2017 still have the same physical vulnerabilities as in 1864. Therefore the Hunley, even though it is a historical case, provides insight into our own physiology that is still applicable to us today. Historical case studies are incredibly important to injury biomechanists because they often present unique scenarios that are unlikely to occur in modern times.” 
 
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 For Lance, the other important aspect of this work was finally being able to solve an enduring maritime mystery. 
 
 “The mystery of the Hunley has been one of the big question marks of history since it disappeared in 1864,” she said. “While the archaeologists of Clemson did the majority of the heavy lifting with their conservation work, it has been a real honor to be able to contribute my expertise to help finally answer the question.” 
 
 [PLoS One] ||||| The submarine H.L. Hunley was the first submarine to sink an enemy ship during combat; however, the cause of its sinking has been a mystery for over 150 years. The Hunley set off a 61.2 kg (135 lb) black powder torpedo at a distance less than 5 m (16 ft) off its bow. Scaled experiments were performed that measured black powder and shock tube explosions underwater and propagation of blasts through a model ship hull. This propagation data was used in combination with archival experimental data to evaluate the risk to the crew from their own torpedo. The blast produced likely caused flexion of the ship hull to transmit the blast wave; the secondary wave transmitted inside the crew compartment was of sufficient magnitude that the calculated chances of survival were less than 16% for each crew member. The submarine drifted to its resting place after the crew died of air blast trauma within the hull. 
 
 Funding: We would like to gratefully acknowledge funding and support from the Josiah Charles Trent Memorial Foundation Endowment Fund at Duke University for directly funding this research. We would also like to acknowledge funding from the DoD SMART Scholarship Program for the research and education of Rachel M. Lance. We also gratefully acknowledge funding provided by the US Army MURI program (U Penn prime—W911NF-10-1-0526) partially supporting Cameron Bass. In addition, we would like to thank the Hagley Library’s Center for the History of Business, Technology and Society for funding via an Exploratory Research Grant that enabled the historical black powder research. 
 
 This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. 
 
 Conventional explosions can injure through one of three generic mechanisms, one of which is primary injury from the blast pressure wave itself (see Ref [ 35 ] for information on other injury types). Primary injuries from blast predominantly affect the gas-filled organs, most often causing pulmonary trauma such as hemorrhaging, but can also present as traumatic brain injuries [ 36 , 37 ]. These injury types would not be evident from skeletal or highly decomposed remains. 
 
 TNT is conventionally used as a yardstick to compare the strengths of different explosives, both low and high, which are typically described by stating their TNT relative equivalency (RE) [ 33 ]. RE is a fractional number that describes the strength of an explosive relative to the strength of TNT. Most explosives have RE values in the range 0.8–1.2 [ 33 , 34 ]. 
 
 Black powder is a low explosive, a volatile blend of crushed charcoal, sulfur, and either sodium or potassium nitrate [ 28 ]. Unlike high explosives, which have burn rates faster than the speed of sound, black powder deflagrates rather than detonates. Variables such as grain size, powder density, and even the type of wood used to make the charcoal can potentially have noticeable effects on powder performance because of their impact on burn rate [ 29 – 31 ]. Tests of modern black powder have shown comparable performance in both burn rate and pressures produced to cannon-grade Union powder from the Civil War [ 29 ]. Black powder performance is also highly dependent upon the strength with which it is confined [ 30 ]. When it is spread on the ground and lit in an open, unconfined environment it burns with negligible pressure generation; however, when it is confined the charge casing allows the gradual generation of internal pressure until the point of casing failure [ 32 ]. While it is categorized as a low explosive, the data presented in this study show that it is capable of generating a sharp-rising pressure wave with certain confinement conditions. 
 
 Momentum transfer into a structure produces motion of that structure, and rapid initiation of motion can create a shock wave off the structure’s back surface [ 22 , 25 , 26 ]. Transmitted blast waves have been observed computationally behind structures and experimentally measured behind armor [ 22 , 27 ]. Computational simulations confirm that backface pressure response is the product of rapid motion of the structure wall in response to the original external blast, and because the motion response of the structure increases for stronger blasts, so will the magnitude of the transmitted blast [ 22 , 25 ]. Such a backface wave means that, even if the original blast wave is largely reflected at the front material interface of the structure, it is possible that people behind the structure could still be injured or killed by transmitted shock from a sufficiently large charge at a sufficiently close range without overt damage to the structure. 
 
 Blast interactions with structures can be prohibitively complex to test experimentally, so these tests are often performed both in air and in water by scaling down the size of the experiment according to the relevant dimensionless parameters [ 17 , 22 ]. G.I. Taylor first described the pi groups that dictate the behavior of an underwater blast wave hitting a solid structure with air behind that structure, primarily in an attempt to predict damage to ships from TNT depth charges [ 23 ]. Taylor’s dimensional analysis and subsequent studies by other groups concluded that the amount of momentum transferred into a structure by a blast wave can be scaled by size if the time-relevant parameters of the blast wave are also scaled [ 22 , 23 ]. Dimensionless parameters dictating the amount of momentum transferred are shown as Eqs ( 3 ) & ( 4 ). 
 
 The increase in both the density and the speed of sound in water means that when explosions occur underwater, the resulting shock and pressure waves travel more efficiently and further than they do in air [ 17 , 18 ]. The most critical behaviors of underwater shock waves follow traditional scaling laws since peak overpressure, duration, and impulse scale with the overall length scale of the experiment [ 17 , 19 ]. However, it is more informative and convenient to describe the output of the charge via the equations provided by Hopkinson scaling, also referred to as the principle of similitude [ 20 ]. The Hopkinson scaling equations for peak pressure and time constant for underwater blast are shown as Eqs ( 1 ) & ( 2 ) [ 17 , 21 ]. 
 
 Both of two previous primary theories of sinking, suffocation and damage to the hull from arms fire, have been found to be implausible in recent publications [ 15 , 16 ]. In addition, evaluation of the attack showed that the Hunley likely drifted before finally sinking [ 16 ]. 
 
 The two large holes discovered in the bow and side of the hull were determined not to be the cause of sinking by analysis of the sediment layers within, which showed that both breaches occurred long after the sinking, and no additional damage was found to the hull that provided an explanation. The holes were determined to have occurred at a later date because analysis of the types and quantity of sedimentary materials, including marine macrofauna, showed strata of sediment deposition that permitted analysis of the general patterns of sediment accumulation over time within the hull [ 13 ]. These strata indicated that the holes were not present during the vessel’s initial time underwater. The pattern of damage of the holes was determined to have been caused by a combination of galvanic corrosion, stresses from riveted seams, and erosion from ocean currents [ 14 ]. 
 
 The Hunley was raised from the ocean floor in 2000, and conservation efforts have been ongoing since [ 5 , 6 ]. The skeletal remains of the crewmembers were found seated at their respective stations, with no physical injuries or apparent attempts to escape [ 7 – 9 ]. The conning towers, which formed the only path of escape, were closed with the aft tower still securely locked [ 10 ]. The bilge pumps were not set to pump out water [ 11 ]. The keel ballast weights, which could be released from within the boat, remained firmly attached [ 1 , 12 ]. 
 
 The vessel’s commander could see out the fore conning tower and was responsible for navigation, while the remaining crewmen powered the vessel’s propeller from the inside using a hand crank [ 2 , 3 ]. At the other end of a hinged 16-foot spar was firmly bolted the Hunley’s torpedo, a copper torpedo of the common Singer’s design type filled with 61.2 kg (135 lbs) of black powder and fitted with a pressure-sensitive trigger ( S1 Fig )[ 4 ]. 
 
 The Hunley sank the Union ship Housatonic and killed 5 Union soldiers by setting off a black powder torpedo against the ship’s hull on the evening of February 17, 1864. The narrow, tapered submarine was 12 m (40 ft) long with a maximum width of only 1.2 m (4 ft) [ 1 ]. It was shaped out of the wrought iron boiler of a previous ship, and carried a crew of 8 men ( Fig 1 ). 
 
 Methods 
 
 Scale model construction A 1/6 length-scaled model of the HL Hunley was constructed out of 16-gauge mild steel, which is materially similar to the wrought iron of the submarine’s hull in many properties including those that dictate structural response to blast exposure (S1 Table) [14, 38, 39]. Key physical design properties of the Hunley were incorporated in the scale model construction, including ballast tanks that could be filled with water and ballast weighting by lining the keel with lead. Information about the methods used to obtain the measurements of the Hunley that formed the basis of the scale model can be found in Ref [15]. The finished model is shown in Fig 2. PPT PowerPoint slide 
 
 PowerPoint slide PNG larger image 
 
 larger image TIFF original image Download: Fig 2. Photograph of the scale Hunley model, nicknamed the CSS Tiny. [a] threaded attachment for spar [b] access port (2 total, one each at bow and stern) to fill and empty the ballast tanks, can be sealed with threaded insert [c] Rings (3 on model) for carrying the vessel and attaching lines [d] Gasket-sealed panel for interior access [e] Data ports (2 on model) for gauges [f] Bulkhead fittings (4 on model) for gauge wires. https://doi.org/10.1371/journal.pone.0182244.g002 The Hunley model, nicknamed the Tiny, was exposed to underwater blasts via three primary experimental methods: shock tube exposures, black powder charges attached to the bow with a size-scaled, angled spar, and black powder charges directed at the side of the hull. For each exposure type, an omnidirectional incident pressure gauge was suspended in the center of the interior of the hull. An identical pressure gauge was also suspended in the water external to the boat, at the centerline along the length of the boat and at a distance of 6 cm horizontal standoff from the side of the hull. The gauges used were oil-filled tourmaline gauges validated for measurement of underwater and air blasts (Naval Surface Warfare Center Carderock Division, Bethesda, MD), amplified with a PCB Piezotronics 402A amplifier and powered with PCB Piezotronics model 482A10 and model 482 power supplies (PCB Piezotronics, Inc., Depew, NY). The gauge cables were foam-covered and insulated along the length of the cable between the boat and the pier. Data acquisition was performed at 1 MHz rate with 500 kHz antialiasing filters using a Hi-Techniques MeDAQ Win600e (Hi-Techniques Inc., Madison, WI). The experiment was scaled so that the degree of pressure transmission into the scaled model hull would be similar to the degree of pressure transmission into the full-sized Hunley [17, 19, 21, 40–42]. Some changes, such as the change in density from salt to fresh water, were experimentally unavoidable, but the effects are expected to be small (differences in density and bulk modulus increase the speed of sound in salt water 2–4% over that in freshwater). 
 
 Shock tube blasts The shock tube exposures were performed in the Duke University Reclamation Pond. The shock tube was composed of a driver section only, made of size 3 high-pressure stainless steel pipe flanges, fitted with a variable number of Mylar membranes (S2 Fig). The shock tube driver was braced from behind with water-saturated wooden rails and pressurized with helium until the Mylar membranes ruptured, creating a shock wave. A live charge creates spherically expanding shock waves, so because the Hunley’s spar held the torpedo at a downward angle the Hunley would have been directly exposed from all sides along the entire length of its hull. Since shock tubes create highly directional shock waves rather than spherical shock waves like those produced by live charges, the shock tube was used to characterize which sections of the hull were responsible for transmitting the effects of blast. The characterization was performed by directing the shock tube at the bow of the vessel, at the side of the vessel, at the vessel’s keel, and at oblique angles relative to the axis of the hull. Once it was determined that the bow of the vessel transmitted negligible blast effects into the main cabin, and the perpendicular component of the blast was responsible for blast transmission into the cabin, the transmission tests were performed by directing the shock tube perpendicularly at the side of the hull. The external pressures were estimated to be the perpendicular component of a blast with the correct direction of propagation, and so were divided by sin(11°) to calculate the overall peak pressure values of the estimated blast [43]. 
 
 Live charge blasts The test site for the live charges was a freshwater pond with a bottom depth slightly greater than the scaled value (9 m/6 = 1.3 m) of the bottom depth at the location of the Hunley attack on the Housatonic [44]. This depth would ensure that reflections of the blast waveform off the bottom would be equal to or less than those experienced by the Hunley, and so would either approximate the amount of bottom reflection or err on the low side since bottom reflections augment the strength of an underwater blast exposure [17]. All necessary permits and legal permissions were obtained prior to each round of testing. Charges were packed with 4Fg black powder (Goex Powder, Inc., Minden, LA) with casings constructed out of schedule 80 PVC pipe with threaded end caps. The historical drawing of the Hunley torpedo indicates that it was filled with grade FF cannon powder (S1 Fig). However, samples of powder from recently uncovered Union cannonballs from the Civil War indicate that the historic FF grain size more accurately matches the modern 4F grain size standard for musket powder [29]. The charges were triggered using NPb squibs (Martinez Specialties, Groton, NY). Isolated squibs were set off to evaluate their detonation signatures at the distances of interest, but were determined not to have a noticeable impact on the pressure waveforms. Several preliminary underwater tests were conducted with variations in charge size, casing construction, and range to the point of measurement to assess black powder’s performance with respect to the scaling of time constant for the non-dimensional groups. Charge size was varied, with sizes of 283 g, 455 g, 490 g, and 1 kg, and range between the charge and the point of measurement was varied between 80 cm and 1.8 m. The range values were selected to validate the scaling principles in the regions most relevant to the Hunley. Initial testing showed that orientation and position of the charge relative to the gauges had a measurable effect on the pressure waveforms. Therefore, test data were only used for the analysis of scaling if they had the same charge orientation and depth as the tests with the scale boat model, and gauge locations that would fall along the length of the submarine hull. Specifically, the tests evaluating the effects of confinement strength were eliminated from the scaling data set because the measurements were taken at the same depth as the charges. The initial time constant of decay (θ) was measured for all blasts. Theta was scaled using Hopkinson scaling by division by the cube root of charge weight (W1/3) (Eq (2)). This value was then plotted as a function of the scaled distance (W1/3/Range) at which the waveform was measured. A power law equation was fitted to the data in the scaling data set using least-squares regression. This method is the standard procedure for describing the time-scaling behavior of explosives and is referred to as the principle of shock wave similitude [17, 21]. The Tiny model was blasted with black powder charges of three sizes: 283 g and 455 g charges, corresponding to 1/6 and 1/5 size scale of the 61.4 kg (135 lb) Hunley torpedo, and 1 kg charges, which were the maximum size as requested by the ATF. While 283 g is the properly mass-scaled value for black powder, the larger charge sizes were constructed to evaluate the degree of propagation of higher pressures through the hull wall. Experimental limitations on scaling burn rate of the powder and methods of charge confinement meant that the PVC 283 g charges would severely underestimate the strength of the exposure compared to the original copper-cased torpedo; larger charges were therefore also tested to evaluate how the transmission properties changed with increases in external pressure. All charges except one were attached via a size-scaled spar to position them in the same manner as the Hunley’s torpedo. One 283 g charge was positioned beside the boat to further increase external pressure of exposure. The external pressure from this charge was divided by the sine of the angle between the direction of blast propagation and horizontal at the centerline of the keel (11°) to calculate the total external peak pressure from a spar-mounted charge that would have the same amount of propagation through the hull. This angular correction for direction of transmission is often used in structural shock testing for charges in different geometric orientations from their targets [43]. 
 
 Shock tube blasting of metal plate Propagation of a sharp-rising shock wave through the full-sized Hunley structure was investigated using a mild steel plate with greater thickness (1.6 cm, 5/8”) than the original Hunley hull (1.0 cm, 3/8”). The purpose of this test was to ensure that the shock wave maintained the sharp rise time critical to cause injuries even when propagating through a material at least the thickness of the full-sized submarine hull. The steel plate was a square 61 cm (24”) on each side and was exposed to airblast using a helium-driven shock tube. The shock tube was 30.5 cm (12”) in diameter and was aimed at the center of the plate. A standoff distance of 4 cm was set between the plate and the end of the tube to allow lateral venting of the shock and provide reduced impulse on the plate relative to peak incident pressure. Incident pressure was measured at the end of the shock tube using 200 psia Endevco pressure gauges (Model 8530B-200, Meggitt Sensing Systems, Irvine, CA) that were flush with the internal wall of the tube body. Two additional Endevco pressure gauges were rigidly fixed behind the center of the steel plate, with 10 cm between the back of the plate and the center of the gauge faces. Both gauges were oriented to measure incident pressure. ||||| Around 6:30 p.m. on February 17, 1864, eight men crammed into the Confederate submarine H. L. Hunley, a self-propelled metal tube attached to a bomb, and slipped quietly into the freezing black water off the coast of Charleston, South Carolina. The crew hand-cranked the sub more than six kilometers toward its target—the Union blockader USS Housatonic—and surfaced like a leviathan for the charge. By 9:00 p.m., it was over: The Hunley had thrust its spar-mounted torpedo into the Housatonic’s hull and within seconds, 60 kilograms of black powder had caved in the ship. 
 
 Just after the brief moment of glory, the Hunley, which had just become the world’s first successful combat submarine, mysteriously sank. 
 
 Its demise has baffled scores of researchers and Civil War buffs for more than a century. Now, one maverick scientist is making the bold claim that she has cracked the case. After three years of sleuthing, Rachel Lance, a U.S. Navy biomedical engineer who holds a PhD from Duke University’s Pratt School of Engineering in North Carolina, concludes that the blast from the sub’s own torpedo sent blast waves through its iron hull and caused instant death for the eight men inside. 
 
 If she’s right, the mystery of the Hunley may finally be put to rest. But how she made the discovery is almost as surprising as the discovery itself: She did it without access to the physical sub, which was excavated in 2000; without prior experience in archaeology or forensics; and without help from the Hunley Project, a team of researchers and scientists at Clemson University in South Carolina that has been on the case full time for the past 17 years. 
 
 Without collaboration or key pieces of data, could Lance’s account of the final moments of the Hunley and its crew be right? 
 
 ********** 
 
 On a warm September Saturday, I’m standing outside the student center at Duke, a low-rise contemporary building accented with the university’s signature neo-Gothic stone, when Lance swings around the bend in a blue Pontiac Grand Prix straight out of Motor City where she grew up. As I open the passenger door to introduce myself, I’m hit by a wall of thumping workout music. Lance just came from the gym, and her brown, shoulder-length hair is thrown up in an elastic. A blue, stonewashed T-shirt that reads Detroit rides up her pale, lanky arms. 
 
 As we make our way off campus, the music keeps pumping. 
 
 “Where are we headed?” I yell. 
 
 “I’m taking you to the campus pond to see where we ran some of our experiments,” she thunders back. “It’s quiet there so we can talk.” 
 
 Lance was modeling an underwater explosion at a computer in Duke’s Injury Biomechanics Lab, where she studied blast injuries, when her adviser had the epiphany that set her Hunley obsession in motion. What if, biomechanical engineer Dale Bass suggested, the modeling software could virtually reconstruct the attack on the Housatonic and reveal insights into the fate of the Hunley? Lance, a history buff, was hooked: a historical mystery with a tantalizing lead to follow. Eventually she’d abandon the software for a more hands-on experimental approach, but Bass’s idea was the catalyst she needed. 
 
 She began reading theories about why the Hunley went down. One prevailing idea was that the crew ran out of oxygen and suffocated. It was exactly the type of theory she was poised to tackle: she’s been a civil service engineer with the U.S. Navy since 2009 and has expertise in breathing system dynamics and, more specifically, rebreathers—the closed-circuit breathing systems divers use to recycle breathing gas underwater. 
 
 As her investigation got underway, Lance noticed there was very little, if any, published research on the crew’s oxygen consumption during the mission. With the navy, she had researched the phenomenon of how much oxygen people used while operating hand-pedal ergometers requiring the same type of motion as the Hunley’s hand-cranked propulsion system. So, she dug up the data and used it to calculate how much oxygen the crew would have used while cranking their way toward the Housatonic. 
 
 It wasn’t clear how much oxygen there was to begin with, though. After hauling up the sub, the Hunley Project conservators calculated how much air was likely available. Their data suggests the crew had enough air for little more than two hours. Lance, however, didn’t have access to the actual data. She had met with project members to discuss collaboration, but they wouldn’t share their calculations with her (and, later on, would ask Lance to sign a non-disclosure agreement, which she’d decline). She’d have to go her own way. 
 
 ********** 
 
 She mulled over the problem for days. Then, she remembered thumbing through a newsletter published by Friends of the Hunley, a nonprofit in Charleston that handles outreach, fundraising, and development for the Hunley Project and runs tours at Clemson’s Warren Lasch Conservation Center where the Hunley is being restored. It was filled with interior and exterior photos of the sub, most of which had measurement notations below them. That gave her an idea. 
 
 For the next month, Lance sat hunched over her desk printing out photos of the sub, measuring each demarcated point with a ruler. After weeks of painstaking work, she finally had all the measurements necessary to calculate oxygen consumption versus supply. The results leapt off the page. Suffocation was not a plausible explanation for why the Hunley sank. 
 
 “Even with conservative calculations, the crew would have been experiencing noticeable hyperventilation, gasping for breath, choking, symptoms of panic, and likely physical pain from high levels of CO2 in the blood,” she says. “But we also know from records that they were seated peacefully at their stations without any signs of struggle. So, from my perspective, this tossed the suffocation theory out the window.” The findings were published in the March 2016 issue of the journal Forensic Science International. 
 
 Richard Moon, the medical director of the Duke Center for Hyperbaric Medicine and Environmental Physiology, agrees. He helped Lance run the calculations and says, “You have a bunch of submariners who were working moderately hard in an enclosed space. There’s no way they would be working away at the crank in a 10 percent oxygen environment with high levels of CO2 and say, ‘Oh well, things are fine; we’ll just keep on going.’” 
 
 The folks at Clemson weren’t convinced. Kellen Correia, president and executive director of the Friends of the Hunley, stated in an email that, “It’s premature to draw any final conclusions about the causes of the loss of the submarine or death of the crew, especially when looking at only one aspect of the situation.” She didn’t, however, reference any specific issues with Lance’s findings. 
 
 ********** 
 
 Debunking the suffocation theory offered Lance some short-term satisfaction, but by this point, she was in deep. She began thinking about the Hunley around the clock, obsessing over it to the point where she’d zone out and stare into her plate of food during dinner with her fiancé. “There was something viscerally terrifying about the fact that eight people died that night, and we had no idea how or why,” she says. 
 
 In the meantime, Hunley Project conservators at the Warren Lasch Conservation Center were chiseling—and continue to chisel—their way through the stubborn, concrete-like layer of sand and silt that formed around the Hunley as it sat on the seafloor for more than 100 years. 
 
 “The de-concretion has the opportunity to give us more information,” says Clemson archeologist Michael Scafuri, “but we haven’t uncovered any definitive evidence to completely explain the loss of the Hunley. Nothing in and of itself explains what happened.” 
 
 There hasn’t been any case-cracking evidence on the human remains side, either. Linda Abrams, a forensic genealogist who has been working on and off with the Hunley Project since 2006, says all of the crew member skeletons were in good shape when they were excavated from the Hunley’s interior. The sub was completely filled with sediment when it was salvaged, so layer upon layer of muck had to be carefully removed before the bones were exposed. “There were no bullet wounds in any of these guys,” she says. And no signs of desperation. 
 
 While the scientists haven’t come up with a smoking gun, there is a small area of damage to the sub’s exterior that has stumped them. The forward conning tower has a softball-sized chunk of iron missing where a viewport had been. 
 
 Through her research, Lance learned of the damage to the conning tower and the so-called lucky shot theory: a stray bullet fired by Housatonic sailors during the attack punctured the tower, causing the sub to fill with water and sink. 
 
 From Scafuri’s perspective, it is a possibility. “The gunfire from the Housatonic may have played a role in this,” he says, “but we cannot confirm that at this point.” 
 
 Lance tested the theory by shooting Civil War-era firearms at cast iron samples—the damage to the sub was inconsistent with damage from her rifle fire. Plus, she says, a bullet hole would have allowed water to rush into the sub quickly and caused it to sink much closer to the attack site than where it was found. 
 
 Based on her results, Lance crossed the lucky shot theory off her list and documented the findings in a second paper in Forensic Science International. 
 
 The Friends of the Hunley declined to comment on the specific findings, but Correia wrote, “Again, Ms. Lance doesn’t have any primary knowledge or data of the Hunley Project.” 
 
 Lance pressed on. If the crew hadn’t suffocated, and a bullet hole didn’t sink the sub, what did happen? 
 
 ********** 
 
 When the Hunley took down the towering Housatonic, it was less than five meters away from the blast. And, it was still attached to the torpedo; inspired by Confederate steam-powered torpedo boats known as Davids during the Civil War, the Hunley’s crew had bolted the sub’s torpedo onto the end of its spar. This meant the same explosion that rocked the Housatonic could just as well have meant lights out for the Hunley crew. 
 
 Lance had spent the better part of two years investigating the suffocation and lucky shot theories, published twice, and still hadn’t solved the mystery. For her, this explosion theory was the next obvious avenue to explore, and one that meshed well with her injury biomechanics focus at Duke. If a blast wave from the explosion propagated into the interior of the sub, she reasoned, it could have immediately killed the crew or at least injured them sufficiently that they would have been unable to pilot the boat to safety. “When blast waves hit an air space, they slow down like a car hitting a wall,” she explains. “Except in this case, the wall is the surface of the lungs.” The sailors’ lungs could have ruptured and filled with blood. 
 
 To test the theory, Lance needed a physical model of the sub. Enter the CSS Tiny, a scale model a sixth the size of the tour bus-length Hunley. Made out of sheet metal, it was a Hunley mini-me right down to ballast tanks filled with water and a steel spar mounted to the bow. 
 
 Engineering a miniature submarine wasn’t a stretch for Lance, who grew up working on old cars with her father, a now-retired GM autoworker. As a kid, she was small enough to slide under their 1966 Mustang to change the oil without jacking up the car. “Growing up around car culture makes it easy to fall in love with machinery and engineering,” she says. 
 
 A few minutes after peeling away from campus in Lance’s Pontiac, we pull into a dusty lot at the Duke University reclamation pond. The thumping bass line cuts out abruptly and the soundtrack is replaced with the ratchet-like chorus of crickets. At the pond’s edge, she gestures to the water, thick with algae: this is where the Tiny took a test run. Lance and a few members from her lab used blast simulation devices known as shock tubes to test the Tiny’s pressure gauges and other equipment in advance of the live explosives phase of the experiment. As she stood in the water, raising and lowering the shock tubes, fish chomped at her legs. It was as if she was being repeatedly stabbed with tiny knives—but by the end of it, Lance and the Tiny were ready for the big event. 
 
 ********** 
 
 The campus pond was off limits to real explosives, so, two weeks later, Lance and her research team trekked out to a three-hectare pond on a rural North Carolina farm for the live ammo tests. They parked the Tiny in the middle of the pond, and with an explosives agent standing guard, the stage was set. Lance began the countdown: “Five! Four! Three! …” The culmination of months of hard work all came down to the next few seconds, and her nerves were frayed as she frantically clicked between sensor readout screens on her laptop. 
 
 From a safe distance, farmer Bert Pitt and his grandchildren were ready for the show. Lance had sweet-talked him into volunteering his pond for the project. “When Rachel came out to the farm,” says Pitt in a thick southern drawl, “she tried to butter me up with red velvet cake and explained that it would only be a one-sixth-scale explosion.” 
 
 “Two! One!” Pfffsssssttt! The black powder charge exploded on the Tiny’s spar, and a small geyser of pond water erupted. Pressure gauges hung inside and outside the vessel to measure the underwater blast waves. Below the surface, the explosion jetted a blast wave into the Tiny’s hull with so much force that it caused the metal to flex. That motion, in turn, generated a second blast wave that transmitted straight through the hull into the cabin. 
 
 “The secondary blast wave from this would have easily caused pulmonary blast trauma that killed the whole crew instantly,” Lance says. “This is what sank the Hunley.” 
 
 Moon supports the conclusion. He says most people would assume that the cabin walls would have protected the crew from the blast waves—but few people know much about underwater explosions. “Speculation up to this point has been fine,” he says, “but when you hold it up to hard science, I think the blast wave theory is the most plausible explanation.” 
 
 While Lance believes the mystery of the Hunley can finally be put to rest, the Hunley Project scientists aren’t ready to jump to conclusions. They’ve acknowledged the explosion theory as a possibility in the past, but began to doubt it prior to Lance’s experiment based on results from a computer modeling study conducted by the US Navy in 2013. The study suggests the blast wave would not have harmed the crew, yet further studies continue to second-guess any previous study conclusions. 
 
 “The problem is, it’s a complicated scenario,” says Scafuri. “It’s sort of like trying to reconstruct the causes of a car accident with limited information. Would you be able to find evidence of an accident that happened because a bee flew in through the window and distracted the driver, who happened to be texting, on a stretch of road that was slick?” 
 
 ********** 
 
 “Oh, I have something for you,” says Lance at Duke’s reclamation pond. She reaches into her backpack and hands me a cigar-sized, 3D-printed replica of the Hunley—a souvenir of sorts. It offers a micro, yet detailed, view of the sub’s interior that makes me realize how confining the crew compartment—which at full-scale was only one meter wide and 1.2 meters high—must have been for eight grown men. It was a death trap. The fact they crammed themselves into the tube anyway was a sacrifice Lance seems to have unwavering respect for. It’s part of what drove her to press on to the finish line, despite the odds being stacked against her. 
 
 ********** 
 
 But how could it be that Lance was able to unravel a century-old mystery in such a relatively short period of time, particularly given the Hunley Project’s 14-year head start? Was it beginner’s luck, or her ability to approach the problem from a different scientific vantage? Maybe it simply came down to old-fashioned determination. “You have to deal with a lot when doing this kind of research, especially when you’re doing things on your own, which can be difficult and lonely,” she says. “You need to have a lot of perseverance, because that’s where the good stuff is—past that limit where nobody’s been able to push through the problem before.” 
 
 In the end, maybe it had more to do with the fact that the Hunley Project is intent on both carrying out the painstakingly slow process of conserving the sub and explaining its disappearance. Although, from a revenue perspective, the mystery in and of itself may be a real positive for the Hunley Project and Friends of the Hunley, considering the sales of T-shirts, shot glasses, and lab tours it helps generate. 
 
 Regardless, when Lance’s findings from her blast wave experiment are published (a research paper will be released imminently), the Hunley Project team will be watching. 
 
 This time, it will be their theory to disprove. 
 
 Related Stories from Hakai Magazine:
– For more than 150 years, researchers have scratched their heads over a Civil War mystery—and now a Navy engineer says she's solved it. Rachel Lance has been diving deep into the 1864 sinking of the Confederate submarine HL Hunley, which mysteriously went down shortly after sinking the Union's USS Housatonic sloop with a torpedo, reports a Hakai Magazine article posted at Smithsonian.com. Over three years, Lance—a Duke grad student with no experience in forensics, no help from experts who'd long been on the case, and no access to the excavated sub itself—carried out what Gizmodo calls an "exhaustive … analysis" and finally arrived at a conclusion published in PLOS ONE: that the torpedo created blast waves that pierced the Hunley's hull and instantly killed the eight crew, whose skeletons were found intact during the sub's excavation near Charleston, SC, in 2000. Lance looked at other theories swirling around the Hunley's fate, including low oxygen or a bullet fired by the Housatonic that punctured the sub. She became obsessed, to the point she'd zone out during meals. "There was something viscerally terrifying about the fact that eight people died that night, and we had no idea how or why," she tells Hakai. Her injury biomechanics background finally led her to her shock wave theory. "When blast waves hit an air space, they slow down like a car hitting a wall," she says. "Except in this case, the wall is the surface of the lungs." A mini replica of the Hunley she dubbed the CSS Tiny and a series of test explosions in a rural pond helped her prove her theory, per CNN. Other researchers remain unconvinced, but Hakai says now Lance's theory is "their theory to disprove." More on her project here. (The Hunley looks a lot better these days.)