A long time ago, something really, really, really BIG hit the Moon. Hard. The explosion was huge beyond human grasp, and when it was all done, the hole it left on the Moon was 900 km (600 miles) across!
Behold, Mare Orientale:
This image was taken by the Lunar Reconnaissance Orbiter's Wide Angle Camera, and shows the entire basin. It's located just over the edge of the Moon as seen from Earth, so we can only get hints of it when we look from home. LRO can see it in all its neck-hair-raising glory.
See all those radial features emanating outward from it? Those are crater chains: secondary impact events as huge chunks of debris hundreds of meters or even kilometers across were thrown hundreds of kilometers away by the force of the impact!
Yegads. You can see these better in an interactive pan-and-scan image that allows you to zoom in to scales of 100 meters per pixel. It's incredible.
But looking at the central part itself, I got a funny familiar feeling. I read reddit, after all. Was the Moon... trolling us?
Well, it's a bit of stretch. I suppose you might see a plain ol' smiley face there, but after spending so much time on teh intertubez it's hard for me not to see the trollface. Or maybe I'm just too sensitized to trolls...
Image credit: NASA/GSFC/Arizona State University