New Evidence Adds to Findings Hinting at Network of Caves on Moon – NASA Science
Like “lava tubes” found here on Earth, scientists suspect that lunar caves formed when molten lava flowed beneath a field of cooled lava, or a crust formed over a river of lava, leaving a long, hollow tunnel. If the ceiling of a solidified lava tube collapses, it opens a pit, like a skylight, that can lead into the rest of the cave-like tube.
