Kola Superdeep Borehole

The Kola Superdeep Borehole (, "Kolskaya sverkhglubokaya skvazhina") was a very deep hole dug on the Kola Peninsula in Russia during 1970-1992 and closed by 2008. It was dug in order to learn more about the inside of the Earth. Digging started on May 24, 1970. The work continued until 1992, or 1994. The borehole was kept open until 2005. SG-3, the deepest part of the hole, reached below the surface in 1989. That is the deepest hole ever made and is still the deepest man-made point on Earth.
For a long time, SG-3 was also the longest hole by distance from its opening. In 2008, the Al Shaheen BD-04A oil well in Qatar reached + farther and, in 2011, the Odoptu OP-11 oil well off Sakhalin got + farther.
Making the hole.
The hole was made with different machines at different times. The Soviets started making the hole with the Uralmash-4E. Later, they used the Uralmash-15000. At first, they wanted to get to below the surface. The machines broke the earlier record for the deepest hole on June 6, 1979. The prior record had been held by the Bertha Rogers hole in Washita County, Oklahoma, in the United States. The machines got to below the surface in 1983. Then the Soviet Union celebrated (took special note of) this for about a year without going any deeper. On September 27, 1984, the machine broke at a depth of ; its drill string twisted off and was left in the hole. When the Soviets started to make the hole again, they had to start from .
The greatest depth was reached in 1989. The people making the hole thought that they would get to by the end of 1990 and, at last, get to 15,000 m by 1993. They were not able to go any deeper, though, because they had made an error, and the end of the hole was in place of the they had hoped for. Their machines stopped making any holes at all in 1992. The last studies were stopped in 2005 and the place was completely closed and left alone in 2008.
What we learned.
The Kola hole got about ⅓ of the way through the Baltic Shield, the part of the Earth's crust (its outer level of rock) below the Kola Peninsula. (People who study the Earth think it's about deep at the place where the hole was made.) The rocks at the bottom of the deepest hole were more than 2½ billion years old.
The people who made the hole said they wanted to learn about the different levels of the Baltic Shield, how sound and heat move under the ground, or what kinds of rocks make up the deep crust, and to create new skills and tools for going deep into the Earth. One of the most important things they found was they had made an error about the kinds of rocks underground. When they looked at how sound waves moved through the Earth, they thought there should be a change from granite to basalt rocks at about below the surface. In fact, the sound waves had changed because the heat and pressure made the granite act differently. Also, this special kind of granite was broken into pieces and full of water. The water did not come from above but had been trapped in the deep crust by solid rock. Another important discovery was that the mud from the deep hole was "boiling" with hydrogen gas.