Lunar Lava Tube

Lunar lava tubes are lava tubes on the Moon formed during the eruption of basaltic lava flows. When the surface of a lava flow cools, it hardens and the lava can channel beneath the surface in a tube-shaped passage. Once the flow of lava diminishes, the tube may drain, forming a hollow void. Lunar lava tubes are formed on sloped surfaces that range in angle from 0.4° to 6.5°._1(https://en.wikipedia.org/wiki/Lunar_lava_tube

The existence of a lava tube is sometimes revealed by the presence of a “skylight”, a place in which the roof of the tube has collapsed, leaving a circular hole that can be observed by lunar orbiters ._3(https://en.wikipedia.org/wiki/Lunar_lava_tube

https://en.wikipedia.org/wiki/Lunar_lava_tube