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Drilled commencing in 1970, the Kola Borehole reached its final depth of 12 kilometers after more than two decades of arduous effort. Scientists aimed to study the Earth’s crust and see how deep they could reach. However, the project ran out of funding in the mid-90s and was abandoned, leaving its impressive record intact. Despite only reaching a third of the Earth's crust, it marks a significant boundary—the final frontier for direct measurements beneath the surface.
The project did fuel speculation, with reports of "voices" emanating from the depths, leading to metaphors connecting the hole to the depths of hell. While it is a somewhat fanciful notion, it emphasizes humanity's intrigue with the underworld.