The first “skylight”, as in big hole in the ground like the amazing ones on Mars, has been found on the Moon by Japan’s Kayuga spacecraft. They are thought to be the collapsed ceilings of lava tubes. There has not been volcanic activity on the Moon for about 2.5 billion years, but it is volcanic activity that created the maria (”seas”) on the Moon, and at one point the entire Moon was a glowing ball of magma.

“Lunar underground”, lunar subterrane, what could possibly be cooler than that? Not much.

A certain Dresden Codak comes to mind.

The new Lunar X-Prize: “Navigate a robotic probe to that hole on the Moon and bring us back pictures!”