Lava tubes can form in two ways: over-crusted tubes form when low-viscosity lava flows fairly close to the surface, developing a hard crust that thickens to create a roof above the moving lava stream. Recently the NASA GRAIL mission provided detailed gravity data for the moon that suggested the presence of enormous subsurface voids related to lava tubes below the lunar maria, plains of basalt formed in volcanic eruptions early in the moon's history. When the JAXA team decided to use their data to try and find lava tubes, they consulted scientists from the GRAIL mission, a NASA effort to collect high-quality data on the moon's gravitational field.
– Thanks to a study in Geophysical Research Letters, we now know where the first human settlement on the moon could be built—a massive lava tube potentially extending miles below the surface. Astronauts haven't been able to spend more than three days on the moon—which lacks Earth's atmosphere and magnetic field—because spacesuits offer minimal protection from radiation, extreme temperatures, and meteorites, according to a press release. A lava tube, on the other hand... "It's important to know where and how big lunar lava tubes are if we're ever going to construct a lunar base," says a researcher at JAXA, Japan's space agency. Researchers at JAXA and Purdue University combined radar data from the SELENE spacecraft and gravitational data from GRAIL to locate what they believe are a number of intact lava tubes—formed when flowing lava forms a crust before draining away—in the Marius Hills area of the moon, the Houston Chronicle reports. Marius Hills is the moon's biggest volcanic dome field. According to Phys.org, Italian researchers recently presented findings that lava tubes on the moon could be massive compared to their counterparts on Earth due to lower gravity, stating the "results have important implications for habitability and human exploration of the moon." One model from the new study shows a lunar lava tube could be large enough to encompass the entire city of Philadelphia. (Russia and the US are building a moon base.)