"We found that if lunar lava tubes existed with a strong arched shape like those on Earth, they would be stable at sizes up to 5,000 meters, or several miles wide, on the moon," said David Blair, a graduate student in Purdue's Department of Earth, Atmospheric, and Planetary sciences. "This wouldn't be possible on Earth, but gravity is much lower on the moon and lunar rock doesn't have to withstand the same weathering and erosion. In theory, huge lava tubes -- big enough to easily house a city -- could be structurally sound on the moon." The researchers applied known...