These structures were discovered off the Japanese island of Yonaguni in 1985. Are they man-made? They resemble the pyramids of Egypt, Mesopotamia, Mexico, and Peru, but analyses showed one was 8,000 years old, which would make these the oldest ruins in the world.

If that’s so, historians can’t explain who would have built them. And archaeologists have suggested that the plates may have formed naturally. For now, the jury’s out.