When most of us think of what humans will leave behind when we go, we imagine skeleton buildings or toxic landfills. But according to some geologists, the longest-lasting impact we'll have on Earth is actually beneath our feet—in the form millions of tunnels, deep boreholes, and mines.
In September 1990, a group of scientists put a drill head to the ground in southern Germany, where two landmasses once merged to form the supercontinent Pangaea 300 million years ago. Their goal? To drill the deepest hole ever made into the earth, a "telescope" into its core.