Lunar Pit

The lunar pits discovered by the Lunar Reconnaissance Orbiter are among the most interesting things ever discovered on the Moon. They are openings in the Lunar surface, skylights looking down into Lunar caverns. I like them because nobody knows how extensive these caves are, or what might be down there, leaving room for the imagination. They tend to be about 100 meters across, with their floors about 100 meters below their rims. Unfortunately, this makes them far too small to be seen with Earth-based telescopes. They are probably ancient partially collapsed lava tubes.

Or, they could have been dug by the Selenites.