For Nautilus, Charles Digges recounts a massive undertaking begun in 1970 by the Soviet Union: As part of its science program, they decided to dig into Earth. Known as the Kola Superdeep Borehole, at 40,230 feet, it was—and is—the deepest human-made hole on the planet. While the hole didn’t go as deep as they planned, the project gave geologists a new understanding of the subterranean world and the earth’s deep continental crust—a place more alive and dynamic than they ever thought possible.

Other discoveries proved just as momentous. Nearly 22,000 feet beneath the surface, drill cores contained microscopic fossils from two dozen species of plankton that had drifted through an ancient sea more than 2 billion years ago. This showed that even after billions of years of burial deep within the continental crust, unmistakable evidence of some of Earth’s earliest life had survived. Between 16,000 and 23,000 feet, the borehole encountered saline water circulating through fractures in crystalline rocks that geologists had long considered essentially dry. Rather than groundwater that had somehow seeped miles underground, the fluids appear to have originated within the crust itself. As ancient minerals were transformed by heat and pressure, they released chemically bound water, while reactions between water and iron-rich rocks generated hydrogen gas, showing that the supposedly inert continental crust was still chemically active billions of years after it had formed.