"An analysis of the stability of lava tubes finds that lava tubes 5 kilometers wide on the moon and 3 kilometers high are perfectly stable," Melosh said. He described how the author of that research, David Blair of Purdue, showed that a lava tube big enough to contain the city of Philadelphia would be stable on the moon.
"That's a little bit of a surprise," Melosh said.
Human explorers haven't set foot on the moon since the Apollo 17 mission in 1972. When astronauts return, lava tubes may help them settle in comfortably. But such sites should be studied and vetted by robotic spacecraft before humans are sent there, Sood and Melosh said.
"We would like to send a radar-based mission," Sood said. "That will give us the possibility to recognize those lava tubes with much more clarity, and potentially find lava tubes that are smaller, that are beyond the resolution of GRAIL."
Hunting for more skylights on the moon would also help, as these features make it easier to explore lava tubes.
"If we already have an access point, that gives you a better chance of going into [a lava tube] than excavating," Sood said.