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Richat Structure, Mauritania

This satellite image shows the Richat Structure. Although it resembles an impact crater, the Richat Structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock.

Planet Observer/Universal Images Group via Getty Images

You've probably seen pictures of Jupiter's mysterious Great Red Spot which actually is a gigantic storm about two to three times the size of our entire planet [source: Choi]. But when viewed from space, Earth has a similarly strange-looking surface feature.

The Richat Structure in Mauritania is a gigantic circular swirl, about 30 miles (50 kilometers) in diameter, that forms a bull's-eye in an otherwise featureless expanse of west African desert. When astronauts first noticed the Richat Structure in the 1960s, it was believed to be a crater left behind by an immense meteor, because of the uniformity of its curves. But scientists now think that it's a rock formation of Paleozoic quartzites, laid bare by erosion [source: NASA]. Either way, though, it's plenty strange looking.

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