A hole in the Sea of Tranquillity, about a hundred metres across, was shown in 2024 to open sideways into a cave — the first lunar cave anyone has proved exists.

Radar from lunar orbit picked out reflections consistent with a conduit tens of metres long running off the bottom of the pit, most likely an empty lava tube. It matters because the surface is a bad place to live — radiation, micrometeorites, 300-degree temperature swings — and forty metres of rock over your head fixes all three at once.