They drilled 12,262 metres and got 0.2% of the way down

On 24 May 1970, on the Kola Peninsula, north of the Arctic Circle, a Soviet team switched on a drilling rig with a round number in mind: fifteen thousand metres down.

Nineteen years later, the branch named SG-3 stopped at 12,262 metres. It is still the deepest point any machine built by people has ever reached. The figure impresses until you set it beside the other one: the Earth's radius is 6,371 kilometres, so twelve kilometres is 0.19% of the way to the centre. They did not even get through the continental crust, which is about thirty kilometres thick there. They stopped at a third of it.

And the well is a needle: at the bottom it measures some 23 centimetres across, barely wider than a sheet of paper, and lowering a tool all the way down took a full day.

What they found tore up the script. Geologists expected a layer of basalt around seven kilometres and met cracked granite instead, soaked in water that had not come from the surface. At six kilometres came microscopic plankton fossils, twenty-four species, intact. And the temperature, which the calculations put at around 100 °C, was 180 °C.

That was why it ended. At 180 degrees rock stops fracturing and begins to flow like modelling clay: the bit turns and does not bite. They stopped in 1992. Today the deepest hole on the planet is capped with a welded metal plate the size of a manhole cover.