Kola Superdeep Borehole
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.