An anonymous reader writes: Astronomers have documented hundreds of holes on the lunar surface. These aren't simply craters, but actual pits ranging from 5 to 900 meters across. Scientists suspects many of these will lead to underground cave systems, which NASA says would be great spots for an astronaut habitat once we get back to the Moon. "A habitat placed in a pit — ideally several dozen meters back under an overhang — would provide a very safe location for astronauts: no radiation, no micrometeorites, possibly very little dust, and no wild day-night temperature swings," said Robert Wagner of Arizona State University. He says it's time to send probes into a few of these pits to see what they're like: "Pits, by their nature, cannot be explored very well from orbit — the lower walls and any floor-level caves simply cannot be seen from a good angle. Even a few pictures from ground-level would answer a lot of the outstanding questions about the nature of the voids that the pits collapsed into. We're currently in the very early design phases of a mission concept to do exactly this, exploring one of the largest mare pits."