We tried to dig to the center of the Earth. π . . . Soviet scientists began drilling the Kola Superdeep Borehole in 1970. After 20 years, they'd reached 40,230 feet β deeper than Mount Everest is tall β but had to stop. The rock at the bottom hit 356Β°F and behaved like plastic, sealing the drill inside. The hole was welded shut in 2005.
Source
BBC Future, 2019, 'The deepest hole we have ever dug' / Smithsonian Magazine, 2015, article on Kola Superdeep Borehole
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