I had no idea that it was once volcanic

ISRO scientists have discovered a giant underground chamber on the moon, which could be used as a lunar base by astronauts for inter-planetary missions. Scientists at the Space Applications Centre (SAC), using data gathered from Chandrayaan-I's Terrain Mapping Camera and Hyper Spectral Imager (HySI) payloads, found a 1.2 km long buried, uncollapsed and near horizontal lava tube.
The hollow cave situated just above the lunar equator on the nearside of moon, can accommodate a large number of astronauts and scientific instruments, and also protect them from hostile lunar environment.
"Such a lava tube could be a potential site for future human habitability on the Moon for future human missions and scientific explorations, providing a safe environment from hazardous radiations, micro-meteoritic impacts, extreme temperatures and dust storms," a team of scientists led by AS Arya of SAC, Ahmedabad said reporting the findings in 'Current Science'.
"Thus lava tubes are not only a safe natural cavity, but also a readymade structure for a lunar base, being a habitat," they said.