This 100-meter hole in the Moon is actually a skylight, the collapsed roof of an underground tunnel carved by lava. These are commonly seen on Earth near volcanoes, but had never been seen on the Moon before LRO was able to map so much of the surface at such high resolution.
The hole itself is also about 100 meters deep. If you were to fall in, it would take about 11 seconds to plummet to the floor, and despite the low gravity you'd impact at about 60 kph (40 mph). Any future astronauts wanting to explore such features - and they will, since these holes give access to parts of the Moon untouched for billions of years - they'd better bring very good spelunking gear!
Original blog post: There's a hole in the Moon!