Media playback is unsupported on your device Media caption Mark Lehner: "A space that the builders left to protect the grand gallery?" 
 
 The mysteries of the pyramids have deepened with the discovery of what appears to be a giant void within the Khufu, or Cheops, monument in Egypt. 
 
 It is not known why the cavity exists or indeed if it holds anything of value because it is not obviously accessible. 
 
 Japanese and French scientists made the announcement after two years of study at the famous pyramid complex. 
 
 They have been using a technique called muography, which can sense density changes inside large rock structures. 
 
 The Great Pyramid, or Khufu's Pyramid, is thought to have been constructed during the reign of Pharaoh Khufu between 2509 and 2483 BC. 
 
 At 140m (460 feet) in height, it is the largest of the Egyptian pyramids located at Giza on the outskirts of Cairo. 
 
 ScanPyramids has already detected a smaller void on the northern face 
 
 The new cavity is perhaps 30m long and several metres in height 
 
 All three muon technologies sense the same feature in the same place 
 
 Khufu famously contains three large interior chambers and a series of passageways, the most striking of which is the 47m-long, 8m-high Grand Gallery. 
 
 The newly identified feature is said to sit directly above this and have similar dimensions. 
 
 "We don't know whether this big void is horizontal or inclined; we don't know if this void is made by one structure or several successive structures," explained Mehdi Tayoubi from the HIP Institute, Paris. 
 
 "What we are sure about is that this big void is there; that it is impressive; and that it was not expected as far as I know by any sort of theory." 
 
 Image copyright SCANPYRAMIDS Image caption The newly found void is directly above the Grand Gallery 
 
 The ScanPyramids team is being very careful not to describe the cavity as a "chamber". 
 
 Khufu contains compartments that experts believe may have been incorporated by the builders to avoid collapse by relieving some of the stress of the overlying weight of stone. 
 
 The higher King's Chamber, for example, has five such spaces above it. 
 
 The renowned American archaeologist Mark Lehner sits on a panel reviewing ScanPyramids' work. 
 
 He says the muon science is sound but he is not yet convinced the discovery has significance. 
 
 "It could be a kind of space that the builders left to protect the very narrow roof of the Grand Gallery from the weight of the pyramid," he told the BBC's Science In Action programme. 
 
 "Right now it's just a big difference; it's an anomaly. But we need more of a focus on it especially in a day and age when we can no longer go blasting our way through the pyramid with gunpowder as [British] Egyptologist Howard Vyse did in the early 1800s." 
 
 Media playback is unsupported on your device Media caption Mehdi Tayoubi: "It's a big void, similar to the Grand Gallery, but what is it?" 
 
 One of the team leaders, Hany Helal from Cairo University, believes the void is too big to have a pressure-relieving purpose, but concedes the experts will debate this. 
 
 "What we are doing is trying to understand the internal structure of the pyramids and how this pyramid has been built," he told reporters. 
 
 "Famous Egyptologists, archaeologists and architects - they have some hypotheses. And what we are doing is giving them data. It is they who have to tell us whether this is expected or not." 
 
 Much of the uncertainty comes down to the rather imprecise data gained from muography. 
 
 This non-invasive technique has been developed over the past 50 years to probe the interiors of phenomena as diverse as volcanoes and glaciers. It has even been used to investigate the failed nuclear reactors at Fukushima. 
 
 Muography makes use of the shower of high-energy particles that rain down on the Earth's surface from space. 
 
 When super-fast cosmic rays collide with air molecules, they produce a range of "daughter" particles, including muons. 
 
 These also move close to the speed of light and only weakly interact with matter. So when they reach the surface, they penetrate deeply into rock. 
 
 But some of the particles will be absorbed and deflected by the atoms in the rock's minerals, and if the muon detectors are placed under a region of interest then a picture of density anomalies can be obtained. 
 
 Image copyright SCANPYRAMIDS Image caption The muon detectors have to be placed under the region of interest 
 
 The ScanPyramids team used three different muography technologies and all three agreed on the position and scale of the void. 
 
 Sébastien Procureur, from CEA-IRFU, University of Paris-Saclay, emphasised that muography only sees large features, and that the team's scans were not just picking up a general porosity inside the pyramid. 
 
 "With muons you measure an integrated density," he explained. "So, if there are holes everywhere then the integrated density will be the same, more or less, in all directions, because everything will be averaged. But if you see some excess of muons, it means that you have a bigger void. 
 
 "You don't get that in a Swiss cheese." 
 
 The question now arises as to how the void should be investigated further. 
 
 Jean-Baptiste Mouret, from the French national institute for computer science and applied mathematics (Inria), said the team had an idea how to do it, but that the Egyptian authorities would first have to approve it. 
 
 "Our concept is to drill a very small hole to potentially explore monuments like this. We aim to have a robot that could fit in a 3cm hole. Basically, we're working on flying robots," he said. 
 
 The muography investigation at Khufu's Pyramid is reported in this week's edition of Nature magazine. 
 
 Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos ||||| Archaeologists have uncovered a mysterious enclosure hidden deep inside the Great Pyramid of Giza, the oldest of the seven wonders of the ancient world. 
 
 The massive cavity stretches for at least 30 metres and lies above the grand gallery, an impressive ascending corridor that connects the Queen’s chamber to the King’s in the heart of the historic monument. It is the first major structure found in the pyramid since the 19th century. 
 
 It is unclear whether the void is a chamber or a corridor, or whether it played any more than a structural role in the pyramid’s construction – such as relieving weight on the grand gallery below. But measurements show that it has similar dimensions to the grand gallery, which is nearly 50 metres long, eight metres high and more than a metre wide. 
 
 Scientists discovered the void using sensors that detect particles known as muons, which rain down on Earth when cosmic rays slam into atoms in the upper atmosphere. The muons travel at close to the speed of light and behave much like x-rays when they meet objects. Armed with suitable equipment, researchers can used them to reveal the rough internal structure of pyramids and other ancient monuments. 
 
 “We know that this big void has the same characteristics as the grand gallery,” said Mehdi Tayoubi at the HIP Institute in Paris, a non-profit organisation that draws on new technology to study and preserve cultural heritage. “It’s really impressive.” 
 
 Tomb raiders: what treasures could lurk inside Egypt's lost chambers? Read more 
 
 Also known as Khufu’s Pyramid, or the Pyramid of Cheops, the Great Pyramid was built in the 4th dynasty by the pharaoh Khufu, who reigned from 2509 to 2483 BC. The monument rises 140 metres above the Giza Plateau and has three chambers known from previous explorations: a subterranean one at the base of the pyramid, the Queen’s chamber at the centre, and the King’s chamber above. While a granite sarcophagus sits in the King’s chamber, King Khufu’s mummy is missing, and his queens were buried elsewhere. Whatever riches were once in the chambers were looted long ago. 
 
 Facebook Twitter Pinterest The ScanPyramids team examining an augmented reality review of the newly-discovered void. Photograph: ScanPyramids mission 
 
 Egyptologists have scores of theories about how the pyramid was built, but there are no reliable accounts of its construction. Herodotus wrote of stones being drawn from quarries near and far, with some being shipped down the Nile on boats. The mammoth construction project occupied the lives of a hundred thousand men, fuelled in part by radishes, onions and leeks, he noted. 
 
 To pinpoint the cavity, scientists from Nagoya University in Japan, and KEK, the country’s high energy physics lab, installed muon-detecting photographic plates and electronic muon detectors around the Queen’s chamber. At the same time, researchers from CEA, France’s energy research organisation, trained “muon telescopes” on the pyramid from the outside. All three techniques can tell from which direction incoming muons arrive. 
 
 When the teams compared their results, all had found a muon hotspot in the same place, indicating the presence of a large cavity in the pyramid. While most of the monument is made of stone that absorbs muons, chambers and cavities let the particles pass through. 
 
 Great Pyramid tombs unearth 'proof' workers were not slaves Read more 
 
 Muon analysis allows scientists to look deep inside ancient monuments without drilling holes or causing other damage to the precious structures. But the technique produces low resolution images, making it impossible for the researchers to tell if the newly-found void runs horizontally or parallel to the grand gallery. Nor can they be sure it is a single enclosure rather than a series of smaller cavities close together, they report in Nature. 
 
 “What we are sure about is that this big void is there, that it is impressive, and was not expected by any kind of theory,” said Tayoubi. To shed more light on the purpose of the cavity, Tayoubi called on specialists in ancient Egyptian architecture to come forward with ideas of what it may be so they can be modelled and checked against the team’s data. The cavity may have relieved weight on the roof of the underlying grand gallery, or be a hitherto unknown corridor in the pyramid. The team has no plans to drill into the cavity to explore inside, but they are developing a tiny flying robot that might one day be sent in, if the Egyptian authorities approve. 
 
 “It’s a tribute to humankind,” said Tayoubi of the pyramid. “It asks a question about what is our future. If they have been able to do this with the means they had 4,000 or 5,000 years ago and they left this heritage today, what will our own society leave for future generations?” 
 
 Peter Der Manuelian, professor of Egyptology and director of the Harvard Semitic Museum, said the discovery was “potentially a major contribution to our knowledge about the Great Pyramid.” 
 
 Facebook Twitter Pinterest Researchers using muon detectors discover a void in the Great Pyramid of Giza. Credit: ScanPyramids mission 
 
 “I’m sure there are imperfections and perhaps small voids or cavities in several locations in the pyramid. What makes this one so interesting is the size, seeming to rival the grand gallery itself in scale,” he said. 
 
 “The muons can’t tell us about chambers, form, size, or any possible objects, so it’s far too early to speculate. I know most people want to know about hidden chambers, grave goods, and the missing mummy of King Khufu. None of that is on the table at this point. But the fact that this void is so large warrants further non-invasive exploration,” he added. 
 
 
 
 In 2011, Rob Richardson, a researcher at the University of Leeds, sent a small snake-like robot into one of the tunnels of the Great Pyramid and took pictures of hieroglyphs that had not been seen for 4,500 years. “I think people assume that all these mysteries of what’s in our world are known but there are still places like the pyramids where we simply don’t know,” he said. “The pyramids have been there for thousands of years and we still don’t know exactly why they are there, what they were used for, or how they were built.” ||||| Scientists Say They've Found Hidden Space In Great Pyramid Of Giza 
 
 Enlarge this image toggle caption ScanPyramids mission ScanPyramids mission 
 
 The Great Pyramid of Giza has been probed with the tools of modern particle physics by scientists who say they have discovered a huge, secret space hidden within its ancient walls. 
 
 It is located above a tall, cathedral-like room known as the Grand Gallery, and this newly found space is comparable in size — about 100 feet long, according to a report in the journal Nature. 
 
 That makes it a major structure within this royal tomb, which was built around 2500 B.C. and is considered to be one of the Seven Wonders of the Ancient World. Yet until now, despite centuries of study, no one knew this space was there. 
 
 Enlarge this image toggle caption Amr Nabil/AP Amr Nabil/AP 
 
 "The romantic interpretation and what everyone wants to hear is that this is a hidden room and the king's body is inside or there's grave goods we didn't know about or we're going to learn more about history ... and none of that is responsible speculation at the moment," cautions Peter Der Manuelian, an Egyptologist at Harvard University who was not part of the research team. 
 
 "All we know is that we have a void, we have a cavity, and it's huge, which means possibly intentional and certainly worthy of further exploration," Manuelian says, noting that it's not yet clear whether it's a single chamber or more than one. 
 
 "In that sense it's obviously frustrating," he says. "On the other hand, as an architectural discovery, something we didn't know about the interior of the Great Pyramid, it's absolutely big news." 
 
 Indeed, the team that made the find reports that it is the first significant internal structure found within the Great Pyramid since the 19th century. 
 
 Mehdi Tayoubi, with the HIP Institute in Paris, explains that he and his colleagues wanted to investigate the pyramid using the best available non-destructive analytical techniques. They settled on a type of imaging that involves muons, which are tiny particles, like electrons. 
 
 "What is strange, for me, is to use those very, very small particles for a huge monument like the pyramid," says Tayoubi. 
 
 Muons are made when cosmic rays from deep space hit the atoms of the upper atmosphere. These particles rain down and lose energy as they pass through materials — like the thick stones of the pyramid — and that makes them slow down and decay. By placing muon detectors in strategic locations, researchers can count the number of muons coming through and create a kind of picture that reveals whether the material above is dense, like stone, or an empty space. 
 
 Tayoubi explains that his team installed sheets of muon-detecting film in a lower-level room of the pyramid known as the Queen's Chamber. The goal was to test whether they could use muons to accurately discern two well-known rooms located above: the King's Chamber and Grand Gallery. 
 
 They saw those rooms but, to their surprise, they found an additional large space as well. 
 
 "The first reaction was a lot of excitement, but then we knew that it would take us a long, long time, that we needed to be very patient in this scientific process," says Tayoubi. 
 
 Because they didn't want to rely on just one method, they confirmed the find using two other muon-detection techniques. 
 
 "The good news is the void is there. Now we are sure that there is a void. We know that this void is big," says Tayoubi. "I don't know what it could be. I think it's now time for Egyptologists and specialists in ancient Egypt architecture to collaborate with us, to provide us with some hypotheses." 
 
 He's interested in whether small robots might somehow enter this space through tiny cracks or holes and provide more information. 
 
 In the past, before the modern science of archaeology evolved, folks sometimes blasted through walls in the pyramids, says Manuelian. 
 
 "That's the good thing about the muon project, there's absolutely no damage to the pyramid at all," says Manuelian. "I hope that, in collaboration with the Egyptian antiquities authorities, further exploration will be set in motion. The study of the pyramids has been going on for an awful long time. So any new contribution is always a welcome addition to our knowledge."
– It's not every day that what is essentially nothingness would be major news, but today is that day: Scientists announced in Nature that after two years of study, they've identified a roughly 100-foot-long void in Egypt's Great Pyramid of Giza—built around 2500 BC, it's the most ancient of the seven wonders of the ancient world, notes the Guardian. It's the first discovery of a new structure in the monument since the 19th century, and it came by using muography, which can pick up on changes in density within rock. What was found, the BBC reports, isn't being called a "chamber," and it explains why: Also known as Khufu's Pyramid, the structure contains voids that are believed to have been incorporated to bolster the structural integrity of the pyramid. While three muon technologies confirmed its existence, "we don't know whether this big void is horizontal or inclined ... [or] made by one structure or several successive structures," says Mehdi Tayoubi. It sits above the pyramid's "most striking" chamber, the Grand Gallery, a corridor that links the Queen's and King's chambers. An archaeologist who reviewed the work says it's possible the void was intended to "protect the very narrow roof" of the gallery, though others contend that for the cavity to effectively have that function, it would need to be smaller. The Guardian notes the pharaoh Khufu's mummy is missing, but a Harvard Egyptologist tells NPR the idea that the cavity is "a hidden room and the king's body is inside ... none of that is responsible speculation at the moment." The team next hopes to get approval to drill a 1.2-inch hole that a robot could fit into. (The world's largest pyramid was mistaken as a mountain.)