Stonehenge, built between 3000 B.C. and 2000 B.C. in Wiltshire, England, is often considered the most mysterious thing on planet Earth. Both the structure's purpose and the method used to build it have puzzled people for millenia.
According to LiveScience, some of the most mysterious things on Earth include Stonehenge, the pyramid city of Teotihuacan, the Great Pyramids of Giza, Cahokia, the Nazca Lines, the Bermuda Triangle and Area 51. The Nazca Lines are a 660 feet ground drawing south of Lima, Peru. The meaning or purpose of the drawings is unknown. In Urfa, Turkey, the Gobekli Tepe is considered mysterious, as archaeologists question how primitive humans were capable of building such a complex structure.