The deepest hole ever dug, and why it stopped
The Kola Superdeep Borehole went 12 km into the Earth, hit rock hotter than anyone expected, and still only got about 0.2% of the way to the center.
On a remote peninsula in the Russian Arctic there’s a rusty metal cap, about the size of a manhole cover, bolted to the ground. Under it is the deepest hole humans have ever made.
The Kola Superdeep Borehole
Soviet scientists started drilling on the Kola Peninsula in 1970. Their goal wasn’t to find oil or gold. They just wanted to see how deep they could go and what they’d find in the crust.
It took almost 20 years. In 1989 the drill reached 12,262 meters, a little over 12 km straight down. The hole is only about 23 cm wide, narrower than a dinner plate.
That’s deeper than the deepest part of the ocean: the Challenger Deep in the Mariana Trench is about 11 km down. But Earth’s center is 6,371 km away, so after two decades of drilling, Kola got only about 0.2% of the way there. (So how do we know what’s down there? Earthquake waves, meteorites and a diamond vise.)
What they found
- It was hotter than expected. The team expected about 100 °C at the bottom. They found around 180 °C. At that temperature, the rock behaved less like stone and more like plastic, flowing back into the hole whenever the drill was pulled out.
- Water, deep down. Water was found kilometers below the surface, squeezed out of the rock itself by the enormous pressure.
- Tiny fossils. Microscopic fossils of ancient plankton turned up about 6 km down.
- No basalt. Geologists had predicted a layer of basalt a few kilometers down. It wasn’t there. The signal they’d seen in earthquake waves came from cracked, water-filled granite instead.
Fun fact: The deepest mine on Earth, the Mponeng gold mine in South Africa, reaches about 4 km down. Kola went about three times deeper.
Why it stopped
The original target was 15,000 meters. The heat made each extra meter much harder than the last, and when the Soviet Union collapsed, the money ran out. Drilling stopped in 1992. The site was later abandoned, and the hole was capped.
Several oil and gas wells drilled since then, in Qatar and off Russia’s Sakhalin Island, are longer than Kola. But they curve sideways through the rock. Kola is still the deepest hole straight down.
How far is that, really?
If Earth were the size of an apple, Kola wouldn’t make it through the skin. A tunnel to the other side of the planet would have to be about 1,000 times deeper than the deepest hole ever drilled.