Skylight on the Moon

Skylight opening on a huge lava tube in the Marius Hills region on the lunar near side.

The Moon's surface is covered by millions of craters, but it also hosts hundreds of very steep-walled holes known as pits. Like doorways to the underworld, photos of some pits clearly show a cavern beneath the Moon's surface, suggesting that they are ‘skylights’ into extensive lava tubes that can be as wide as New York's Central Park, and could extend for hundreds of kilometres. These tubes are thought to have formed during lava flows billions of years ago, when the Moon was still geologically active.