The Kola Superdeep Borehole

Plankton fossils six kilometres down, and the hole is just capped

The Kola Superdeep Borehole, SG-3, in the Pechengsky District of the Kola Peninsula near Russia's border with Norway, is the deepest hole people have made: 12,262 metres, reached in 1989. It is only 9 inches across. The Soviet Union drilled it to study the Earth's crust, not to find oil or gas. Work began on 24 May 1970 with an ordinary oil-well rig aiming for 7,000 metres; in 1974 a purpose-built Uralmash-15000 rig arrived with a new target of 15,000 metres, and on 6 June 1979 SG-3 passed the world record held by the Bertha Rogers hole in Oklahoma. Instead of turning the whole drill string, the Soviets turned only the bit, driven by the flow of drilling mud.

It was never one straight hole. In September 1984, at 12,066 metres, a 5-kilometre section of drill string twisted off and was left in the hole, and drilling restarted from higher up. The third branch reached 12,262 metres in 1989. A fourth was stopped in 1992 at 11,882 metres by temperatures of 180 °C, 356 °F against the 212 °F that had been mapped, and the hotter the rock, the harder it was to keep the bore open. A fifth was abandoned in 1994 for lack of money.

What came up overturned the predictions. The basalt layer expected at 7 kilometres was not there, nor at any depth, only more granite, and the seismic boundary turned out to mark a change within metamorphic rock. Between about 3 and 6 kilometres the rock was fractured and saturated with water that had been forced out of the minerals themselves and trapped under impermeable rock. The drilling mud came up described as boiling with hydrogen gas, and microscopic plankton fossils turned up 6 kilometres down. The bottom of the hole is in rock about 2.7 billion years old, roughly a third of the way through the crust of the Baltic Shield.

Drilling ended in 1995 when the funding stopped. The scientific team was dissolved in 2007, the private company that took the equipment was liquidated in 2008, and the site was abandoned. Sightseers still make the trip and find the structure over the hole partly destroyed or removed.