Science Puzzle
How Deep Is the Deepest Hole Ever Dug?
In 1970, Soviet scientists began drilling a hole straight down into the Earth on the Kola Peninsula in the far north of Russia, to find out what the deep crust is really like. They kept going for more than 20 years. It's still the deepest hole people have ever made.
How deep did they get?
The Answer
About 12 kilometres. In 1989 the Kola Superdeep Borehole reached 12,262 metres, deeper than Mount Everest is tall, through a hole only 23 centimetres wide.
They planned to go deeper, but the Earth got too hot. Rock gets warmer the deeper you go, because heat flows outwards from the planet's hot interior. The scientists expected about 100 °C at the bottom. They found 180 °C. At that temperature the rock was soft enough to slowly creep into the hole and jam the drill, and in 1992 the drilling stopped.
Even so, it's a scratch. Twelve kilometres is only about a third of the way through the continental crust in that region and about 0.2 per cent of the way to the centre of the Earth, some 6,370 kilometres down. On a globe the size of a football, the hole would be thinner than a hair and too shallow to feel.
It still found surprises: hot, salty water circulating in cracks far deeper than anyone expected, and tiny fossils of plankton about 6 kilometres down.
The principle: The geothermal gradient. Rock gets hotter the deeper you go, so at great depths it becomes too hot and soft to drill, which limits how far into the Earth we can reach.