Deepest penetration into the Earth's crust

The deepest penetration into the Earth's crust is a geological exploratory borehole near Zapolyarny on the Kola peninsula of Arctic Russia. It was begun on 24 May 1970 and reached a depth of 12,261 m (40,236 ft) by 1983, when work stopped due to lack of funds. The temperature of the rocks at the bottom of the hole is about 210°C (410°F).

The object of the Kola hole wasn't oil or gas extraction but first-hand knowledge of the nature of the Earth's interior. In particular the Soviets wanted to reach the "zone of discontinuity" detected by geophysicists monitoring seismic waves traveling through the Earth's crust, thought to be where granite layers meet molten basalt. The researchers met some surprises, discovering the discontinuity zone was actually at the base of an area of fractured metamorphic rock, about 9 km (6 miles) underground, saturated with liquid water – something never previously suspected as existing so deep.

An American rival project was the 1961 Mohole Project, based off the Pacific coast of Mexico, but funding ran out in 1966.

In 1971 oil prospectors in Oklahoma sank a shaft 9,583 m (31,441 ft) deep, but had to abandon going any deeper when they hit molten sulfur.

A German scientific project known as the KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik) Hole, based near Windischenbach had drilled down as far as 9,101 m (29,858 ft) as of 2000.