Under this metal cap is the world's deepest hole
J Thoendell stashed this in Science
The most intriguing discovery made by the Kola Superdeep Borehole researchers was the detection of microscopic plankton fossils four miles beneath the surface of the earth. Usually fossils can be found in limestone and silica deposits, but these "microfossils" were encased in organic compounds that remained surprisingly intact despite the extreme pressures and temperatures of the surrounding rock.
Drilling at Kola stopped in the early 1990s when scientists encountered prohibitively high temperatures. The Superdeep Borehole is still the superdeepest human-made hole on the planet.
Today I learned the world's deepest hole is 7.5 miles deep:
While American researchers faltered with Project Mohole, a dig off the coast of Mexico that ran out of funding in 1966, their Russian counterparts took a more determined approach. From 1970 to 1994 their drill on the Kola Peninsula burrowed through layers of rock, reaching an ultimate depth of 7.5 miles. (The distance to the center of the earth is around 3,950 miles, but the continental crust is a mere 22 miles thick.)