It’s the deepest man-made hole in the world: over seven miles.

Soviet Russia wanted to go deeper. By the time it closed up shop in 1994, the Kola Superdeep Borehole had worked its way through one-third of the continental crust. Even after pioneering new methods of drilling, that was as far as they could go: after a certain point, the earth’s molten crust acts almost like liquid plastic, closing up any hole they would make.

They took core samples all the way along, and geologists were agog. Miles below the surface, they found water – but not free water, like you can find in any earthly ocean. Instead, the ungodly pressure of the earth just squeezed together hydrogen and oxygen atoms – until water molecules had no choice, but to exist. They also kept on finding fossils – four miles down.

America once tried to do the same: they tried drilling straight into the sea floor, under two miles of water. They only got six hundred feet. The project was known simply as, ‘Project Mohole.’