Remove this rusted metal cap and the world’s deepest hole tunnels miles into the Earth. However, we know more about certain distant galaxies than we do about what lies miles beneath our very own feet. For that reason, Soviet scientists in the 1970s decided to probe deeper than humanity has ever done before. For the next 24 years, they drilled on and off into the Earth’s crust.

The result was the Kola Superdeep Borehole and a drill-depth of more than 7.5 miles (12 kilometers). To put that in perspective, Kola descends further than the deepest point of the ocean, which lies at nearly 6.8 miles (11 kilometers). The borehole is located on the Kola Peninsula of Russia.

So did we learn anything from these decades of labor? Thankfully, yes! Scientists found microscopic fossils of single-celled organisms at 4.3 miles (7 kilometers) down. And at nearly the same depth, they discovered water. They also found that the temperature at the bottom of the hole reached a blistering 356°F (180°C). Too hot to continue, drilling officially halted in 1994.

However, what’s even more impressive is that scientists estimate that the distance to the center of the Earth is nearly 4,000 miles (6,400 kilometers). Turns out, 7.5 miles barely scratches the surface.

Watch a video about the Kola Superdeep Borehole presented by Hank Green from SciShow below: