Humans are curious by nature. We’ve looked outward and explored space, but rarely have we looked at what’s right beneath our feet. And in the most ambitious project to date, it took 24 years for humanity to penetrate a mere 7.5 miles into the Earth’s surface.

The Soviets created the Kola Superdeep Borehole in the 1970s in western Russia, so scientists could learn more about the Earth’s crust.

Over two decades, they managed dig more than 7.5 miles down into the Earth. In 1992 the project was abruptly stopped, however, because the temperature down below reached over 350 degrees Fahrenheit — far hotter than the scientists predicted it would be. They were unable to keep drilling, as the heat would have destroyed their equipment.

Discoveries

Alexander Tumanov/TASS/Getty

Though work had stopped, some scientific discoveries were indeed made. The researchers learned there was water that far down into Earth’s crust, despite previously assuming this was impossible. They think it may be formed from stray oxygen and hydrogen atoms squeezed out of rock minerals. Scientists also discovered 24 new types of long-dead single-celled organisms. They also gained access to rocks that were 2.7 billion years old. But even after decades of digging, this hole only went a fraction of the way to the center of the Earth.

The hole today

The Kola Superdeep Borehole, pictured in the video below, is nine inches in diameter. It’s covered with a metal lid, which is welded on to prevent an unfortunate passerby from falling in. If you fell down the hole, it would take around four minutes to reach the bottom. Locals have nicknamed the hole “The Well to Hell,” and claim they can hear the screams of those being tortured down below. Just as well they put a lid on it.

You can learn more about the deepest hole in the world here: