On the Kola Peninsula, in the far north-west of Russia, near the Norwegian border, there is a rusting metal cap bolted to the ground in the ruins of an abandoned research station.

It's about the size of a manhole cover. Underneath it is the deepest hole humans have ever made.

It goes down 12,262 metres. More than twelve kilometres. If you dropped Mount Everest into it, the peak would be more than three kilometres below the surface.

And yet, on the scale of the planet, it barely scratches the paint.

The race to the bottom

In the 1960s, as the US and the USSR raced into space, a quieter race was going on in the other direction.

Scientists knew a great deal about the stars and almost nothing direct about what was under their feet. Everything we knew about Earth's deep interior came from indirect methods, especially from studying how earthquake waves travel through rock. Nobody had ever actually been down there.

The Americans tried first, with a project to drill through the thin oceanic crust, but it was cancelled in the 1960s. The Soviets took a different approach: drill on land, through the thick continental crust, as deep as they could go.

Drilling began on 24 May 1970.

A very narrow hole

The borehole isn't a shaft you could climb down. Its main branch is only about 23 centimetres wide — roughly the width of a dinner plate.

The drill bit was turned not by spinning the whole kilometres-long pipe, which would have twisted it apart, but by a turbine at the bottom, driven by drilling mud pumped down from the surface. Every time a bit wore out, the entire string of pipe had to be pulled up, section by section, and lowered down again. Near the bottom, a single round trip could take most of a day.

Several branches were drilled from the main hole over the years. In 1989, one of them, known as SG-3, reached 12,262 metres.

What they found

The hole was supposed to answer questions. It mostly raised new ones.

The rock was wet. Deep in the crust, where scientists expected solid, dry granite, the drill encountered water. It wasn't from the surface. Researchers concluded it had been squeezed out of minerals by immense pressure and then trapped by an impermeable layer of rock above.

The expected basalt wasn't there. Earthquake data had suggested that below a few kilometres, granite would give way to basalt. The drillers went well past that depth and found no basalt. The change in the seismic signal, it turned out, was caused by granite that had been transformed by heat and pressure — not by a different type of rock.

There were fossils. About six kilometres down, scientists reported microscopic fossils of single-celled plankton — preserved in carbon and nitrogen coatings rather than limestone.

The mud seemed to boil. At great depth, drilling mud came back up thick with hydrogen gas.

The scream from hell

Here's where the Kola borehole took on a second life — as one of the internet's most durable legends.

In 1989 and 1990, a story began circulating through Christian newsletters and broadcasts. Soviet scientists, it claimed, had drilled 14.4 kilometres into the Earth "in Siberia", broken into a cavity with a temperature of 1,100 °C, and lowered a heat-proof microphone. They heard the screams of the damned.

Deep, but not deep

Twelve kilometres sounds enormous, and in human terms it is. It's deeper than the deepest point of the ocean, in the Mariana Trench, at about 11 kilometres.

But Earth's radius is about 6,371 kilometres. The Kola borehole made it about 0.2% of the way to the centre. If Earth were an apple, the deepest hole humans have ever drilled wouldn't break the skin.

The hole today

The research station at Kola is now a ruin of broken windows and rusting machinery, visited by urban explorers and the occasional curious tourist. The hole itself is sealed under its small, bolted metal cap.

It's an oddly modest monument to one of the most ambitious scientific projects of the 20th century: a pinprick in the Arctic that went further into the Earth than anything else ever has.

And, if you put your ear to the cap, you will hear absolutely nothing at all.