In the Russian Arctic, roughly 10 kilometers from the Norwegian border in the Pechengsky district of Murmansk Oblast, there is a sealed metal cap about 23 centimeters across, bolted and welded into a concrete slab. It covers the Kola Superdeep Borehole, the deepest hole ever drilled into the Earth. The shaft descends more than 12 kilometers into the planet’s crust.

The project began on May 24, 1970, during the Cold War, when the Soviet Union set out to drill 15,000 meters into the Earth. The stated goal was to reach a boundary in the crust that geological models predicted lay at roughly 7 kilometers, where granite was expected to give way to basalt. A research station was built at the site, and the work proceeded for decades.

By 1989, the deepest branch of the hole, known as SG3, reached a depth of 12,262 meters. To put that in context, that is deeper than Mount Everest is tall and deeper than the Mariana Trench. The drill column could not be spun from the surface, as 12 kilometers of steel would twist apart. Only the drill bit at the bottom rotated, while the column above hung motionless.

The project stopped at that depth, which remains the record for borehole depth. The reason was practical: at 12 kilometers down, the rock was far hotter than predicted. Sensors measured 180 degrees Celsius, nearly double the expected 100 degrees. At that temperature and under that pressure, granite softened and flowed like plastic. When the drill bit was pulled back, the shaft would begin to close, narrowing and deforming as the rock pressed inward. Drill heads came up twisted, and pipes snapped under the strain, forcing the crew to abandon branches and start new ones.

The conditions also produced several scientific surprises that contradicted prevailing geological models. The expected transition from granite to basalt never appeared. The Conrad discontinuity, a boundary derived from seismic surveys and accepted as standard in geology, could not be located. The drill passed through fractured granite for the entire depth, with no basalt at all.

At depths where rock was expected to be dry and sealed, the drill found water. It had not seeped down from the surface. The water was presumably squeezed out of the minerals themselves by heat and pressure, collecting in fissures in the rock. Hydrogen gas also rose through the drilling mud in such volume that the crew described the mud as boiling.

In cores of rock roughly 2 billion years old, drilled at around 6 kilometers deep, scientists found microscopic fossils. The mineralized shells of approximately two dozen species of ancient plankton were preserved intact, with their carbon and nitrogen content still present, despite the heat and pressure that should have destroyed them.

The story that later attached itself to the borehole originated in 1989, when a U.S. television network, Trinity Broadcasting, aired a claim on the project. The report said Soviet engineers had broken into a hollow cavity and lowered a heat-resistant microphone into it, and had recorded human voices screaming in agony. The network cited a Finnish newspaper and quoted a scientist named Dr. Azzacove, who supposedly described temperatures of 1,100 degrees Celsius and the cries of the damned.

None of that was accurate. The Finnish source was a small Christian newsletter that had reprinted a reader’s letter. Dr. Azzacove appears nowhere on the project’s staff lists and is believed to have been invented. A Norwegian school teacher, Aage Rendalen, heard the broadcast and, in an attempt to discredit it, wrote to the network with an even more elaborate version of the story, including a claim about a bat-winged creature emerging from the hole. The network aired his fabrication as well. Audio clips circulated online as supposed recordings from the hole were later traced to screams taken from a 1972 Italian horror film, Baron Blood.

The real project ceased drilling in 1992, following the collapse of the Soviet Union and the end of funding. The science station operated on skeleton staffing for a few more years before closing in the mid-1990s. By 2008, the site was abandoned. Scrappers stripped the equipment, the derrick was removed, and the open hole was capped with a metal plate welded into concrete.

The program’s director, David Guberman, commented on the rumors at one point. He noted that he had heard the stories about screams and had no full explanation for how they started, but he stated plainly that what was down there was not hell.

Later attempts to drill deeper elsewhere have not exceeded Kola’s depth. Germany’s KTB Borehole in Bavaria reached about 9,000 meters in 1994 before heat and twisted tools ended the effort. Some oil wells have drilled longer horizontal distances, and Japan’s research ship Chikyu has pursued the goal of reaching the mantle but has not yet done so. No team has returned to the Kola site. The cap remains in place, and the borehole still holds the record as the deepest point humans have reached beneath the surface of the Earth.