6. Eye of the Sahara, Mauritania
Perhaps more accessible to astronauts than to your average holidaymaker, the Eye of the Sahara (also known as the Richat Structure) is the geological equivalent of an absolutely massive crop circle. Measuring 28 miles in diameter, this extraordinary phenomenon is apparently an ‘eroded, uplifted geologic dome’ (nope, we’re not sure what that is either). There would definitely be theories about UFO landing sites if it was better known.