A rusted iron plate roughly the size of a manhole cover sits welded shut on a concrete pad in the far north of Russia. Beneath it is a hole twenty-three centimetres wide and twelve and a quarter kilometres deep, the deepest anyone has ever reached into the planet, and it was abandoned because the rock at the bottom stopped behaving like rock.
The short version
- Drilling started in 1970 and stopped at 12,262 metres in 1989. The hole is still the deepest point ever drilled below the surface.
- It was abandoned because the temperature hit about 180 degrees Celsius, nearly double the prediction, and the rock became plastic.
- The genuine surprises were water, hydrogen and microfossils at depths where none were expected.
- The recording of screaming voices from the hole is a hoax built from a horror film soundtrack.
What it was for
The project was Soviet science at its most ambitious. In the 1960s both superpowers wanted to sample the deep crust directly rather than infer it from seismic waves, and the Americans abandoned their attempt off the Pacific coast. The Soviet team chose the Kola Peninsula, near Zapolyarny in Murmansk Oblast, where the exposed Baltic Shield offered ancient continental crust with no thick sediment cover to drill through first.
Work began on 24 May 1970. Rather than one shaft, the project drilled a main hole with several branches; the branch designated SG-3 reached 12,262 metres in 1989. At that depth the bore is only about twenty-three centimetres across, which is worth holding onto: this was a needle, not a shaft, and nobody was ever going down it.
Why it stopped
Predictions put the temperature at around a hundred degrees Celsius at target depth. The actual reading was closer to a hundred and eighty. Hot rock under enormous confining pressure stops fracturing cleanly and starts flowing, so the walls of the hole crept inward and the cuttings behaved like plasticine. Drill bits failed faster than they could be replaced and sections of string were repeatedly lost. Attempts to push past 12,262 metres kept curving off course.
Funding thinned after 1992 and the research station closed in the mid-1990s. The site was formally abandoned in 2008, the equipment scrapped, and the hole capped and welded. The buildings around it are a ruin.
The results that mattered
No basalt layer
Seismologists had interpreted a boundary a few kilometres down, the Conrad discontinuity, as the top of a basaltic lower crust. The core told a different story: the same granitic and metamorphic rocks continued, but their density and seismic velocity changed because of increasing metamorphic grade and fracturing. A boundary that had been read as a change of material turned out to be a change of condition. That single result rewrote a standard model of the continental crust.
Water where there should be none
Free water was found circulating in fractures many kilometres down, in crystalline rock that was assumed to be effectively dry. The interpretation is that it is not rainwater that has seeped in, but water squeezed out of the minerals themselves and trapped by impermeable rock above.
Fossils and gas
Microscopic fossil remains of single-celled marine organisms were identified in core from roughly six kilometres down, well preserved in rock two billion years old. The drilling mud also came up saturated with hydrogen, in some accounts boiling with it. Both findings were unexpected and both have held up.
| Detail | Figure |
|---|---|
| Drilling started | May 1970 |
| Maximum depth | 12,262 metres, 1989 |
| Diameter at the bottom | About 23 centimetres |
| Temperature at the bottom | About 180 degrees Celsius |
| Continental crust thickness here | Roughly 35 kilometres |
| Proportion of Earth's radius | About 0.2 per cent |
| Sealed and abandoned | 2008 |
The sounds of hell
The story that microphones lowered into the borehole picked up the screams of the damned has circulated since the late 1980s, spread first by a small newspaper item and then by religious broadcasters. The audio attached to it is not a field recording. It is a heavily layered and reversed edit of an orchestral horror film soundtrack, and the layers can be separated back out. No microphone survives 180 degrees at that depth, no such experiment appears in any project record, and the Soviet team never claimed one.
It is worth noting why the hoax was so effective: the real project genuinely did stop early, genuinely did find things nobody predicted, and genuinely was sealed with a welded plate. Every element of the legend has a true fact underneath it, rotated slightly.
Confirmed
- Depth of 12,262 metres, still unmatched vertically.
- The absence of a basalt layer.
- Deep circulating water and abundant hydrogen.
- Microfossils at around six kilometres.
Not real
- Recorded voices or screams.
- A cavity, chamber or void at the bottom.
- Any suggestion the hole reached the mantle.
- Claims that it was sealed to hide something.
How deep is deep
The number that puts the achievement in perspective is unflattering. Twelve kilometres into a planet with a radius of 6,371 kilometres is about two tenths of one per cent. On a standard classroom globe the hole would not break the varnish. The continental crust here is roughly thirty-five kilometres thick, so the drill was still less than a third of the way through the outermost layer.
Longer holes exist now, drilled for oil, but they run mostly sideways. Measured straight down from the surface, nothing has beaten a Soviet rig that ran out of money in 1989, and the reason is not ambition. It is that heat wins.
About the author
Motech researches and produces long-form video on technology and the unexplained for an audience of more than 270,000 subscribers. Corrections and tips: info@richardelhaj.media