4. The Devil’s worm is one of the weirdest. Measuring just half a millimetre, it was found at a depth of 1.3 kilometres, or 0.8 of a mile, in a South African gold mine where temperatures reach 37C. The worm is the deepest living multicellular terrestrial organism on Earth. It was named Halicephalobus mephisto after Mephistopheles, the demon in the Faust legend.