And some pits on the surface could be skylight-like entrances to the lava tube system below. The LRO’s camera first confirmed the presence of pits that could potentially lead to caves in 2012.

This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's Narrow Angle Camera is 400 meters (1,312 feet ...