The deepest hole humanity has ever dug is the Kola Superdeep Borehole. Soviet scientists, hoping to learn about the composition and geophysics of the Earth’s crust and upper mantle, began drilling on the Kola Peninsula, in the country’s extreme northwest, in 1970. After more than two decades, they reached a depth of 12 kilometers: a slender borehole three times the length of Central Park, and 113 times the depth of the world’s deepest metro station (Arsenalna in Kyiv).
The hole is less than 0.2 percent of the way to Earth’s core, whose center is thought to be 5,200 degrees Celsius. That temperature isn’t far off being twice as hot as the temperature necessary to vaporize iron. Knowing this, the scientists expected the bottom of their 12-kilometer hole to be 100 degrees Celsius, hot enough to boil water. Instead, it was 180 degrees Celsius, the heat of an oven.
At those depths, the drilling was pushing the limits of technological possibility. It’s difficult to get good performance out of a drill bit that far from the surface. It’s hard to dispose of ‘cuttings’ – the rock torn up by the drill – and harder still to keep replacing those drill bits, which quickly get gnarled up, without the swapping process leading to increasing inefficiency as the wellbore deepens. In these extreme conditions, granite behaves more like plastic than rock. It became more and more challenging to drill, and the project ran out of money. In 1995, the nine-inch borehole was welded shut.