Lava tube

A Lava tube forms when a volcanic flow cools at the surface, forming a solid crust while remaining fluid inside, allowing the lava to continue flowing. When the flow of molten lava stops, the tube empties.

The size of these tubes is very variable. Thus, the Kazumura, in Hawaii, at 59.3 km, is the longest tunnel known on Earth.. Those on Iceland, Reunion island and, of course, Hawaii are famous.

A lava tube with a partially collapsed ceiling in Hawaii forming what is called a skylight. © USGS

When the ceiling of one of these tunnels collapses letting in the light, it is referred to as a skylight . When this occurs over a long distance it is referred to as a "sinuous rille". There are many of these rilles on the Moon.