As much as I like gazing at the Moon, it seems pretty cold and dead. But apparently it had an exciting past. NASAs GRAIL spacecraft have seen evidence of a giant square-shaped volcano under the surface. Researchers think it's 3.5 billion years old.
What's interesting is that NASA found this site not by looking at photos but by mapping the Moon's gravity. The GRAIL probes sense the pull in different areas and noticed this square tugged harder than surrounding regions. The mission is soon coming to an end, so this was quite a lucky find.