The deepest hole ever drilled is the Kola Superdeep Borehole, on the Kola peninsula in the northwest corner of Russia, located near Finland. It was drilled for scientific research by the USSR. Like many large research holes, it had a number of offshoots from the central branch, and the deepest, SG-3, was 7.6 miles (12.262 km) deep. The borehole reached this depth in 1981. The temperature at this depth was 356°F (180°C), at which point the rock became more like a plastic than a solid, stopping further drilling.
Even though the hole in question was 7.6 miles (12.262 km) deep, it only penetrated a third of the Baltic continental crust. The rock at the bottom of the hole was about 2.7 billion years old, and samples brought from near the bottom contributed invaluably to the study of geology and geophysics at the time. The original depth goal was 9.32 miles (15,000 meters), but a faster-than-expected increase in temperature forced a premature halt to the project. If the hole had extended down to its planned depth, the projected temperature would have been 572°F (300°C), well over the maximum operating temperature of the drill bit.
The Kola Superdeep Borehole was the source of a tabloid rumor, started by a Finnish newspaper, that Russian researchers had burrowed through to Hell. The story was reproduced by several American tabloids. It stated that 9 miles (14.4 km) down into the Earth's crust (1.4 miles (2.25 km) deeper than the real depth), the scientists reached a pocket of air with a temperature of 2,000°F (1,093°C). Intrigued, they sent down a heat-tolerant microphone, which picked up the screams of the damned. The rumor was exacerbated when recordings of the alleged screams popped up on the Internet shortly thereafter.
Several interesting discoveries resulted from the hole. Pockets of water were found from time to time, prevented from ascending to the surface by a layer of impermeable rock. An excess of hydrogen was released from the drilling, which researchers described as "boiling" up from the surface of the hole. More details on the geology of the Baltic Shield were discovered. This area formed hundreds of millions of years ago when a supervolcano exploded and sent magma across a fair fraction of the continent.