Billions of years ago, rivers of runny basalt lava crossed the Moon. Where their surfaces crusted over, the lava kept flowing underneath and later drained away, leaving hollow tubes. Orbiters have photographed more than 200 pits on the Moon. A few are skylights where a tube's roof fell in, and through them we can see into these caves.
Each photo is from orbit and pinned to its real location. Select one to see it full size.
An illustrative model of a tube beneath a collapsed roof, loosely scaled on the Marius Hills and Mare Tranquillitatis pits. Move the Sun to see the light patch shift across the floor. With no air, sunlight makes no visible beam, only a lit spot where it lands.
Mostly unknown, because nothing has been inside one yet. The surface gives clues:
Probably one main tube, not a maze. Lava tubes follow a single stream of lava downhill, unlike limestone caves that dissolve along every crack. On Earth they often have:
On the Moon, rilles and pit chains look like single lines. The rille inside Vallis Schröteri and the layered walls of the Marius Hills pit show that lava flowed through the same areas more than once.