The drilling machine in a ship had observed something above a spot in the Indian Ocean. They began drilling toward the mantle. The scientists work for the International Ocean Discovery Program. They planned to bore through six kilometres of tough oceanic basalt – the Earth’s crust – and then pierce the mantle.

No one has ever drilled into the mantle before, but there have been half-dozen serious attempts.

Decades ago, the Russians drilled deeper than anyone has ever gone.

Their Kola Superdeep Borehole was started in 1970 and still holds the world record for the deepest hole in the ground. But they didn’t reach the mantle. As the recent mantle drilling project began on the coast of Africa, people were wondering if a billion dollars for the newest hole in the ground is worth the money.

In actuality, the Kola Superdeep Borehole consists of several holes branching from one central hole. The deepest of these, named “SG-3”, measuring nine inches in diameter but extends 12,261 meters (or 7.5 miles) into the Earth. That’s roughly a third of the way through the Baltic continental crust.

To compare, the deepest ocean in the world is located in the Pacific Mariana Trench “Challenger Abyss”, about 11,000 meters deep. It can be seen that the Kola Superdeep well is really very deep.

To meet scientific objectives and provide a nearly continuous look at the crust’s profile, the Soviets even developed instruments to take direct physical measurements at the bottom of the borehole. The drilling apparatus thus allowed for greater measurement integrity since rock samples would deform under their incredible internal pressure when brought to the surface.

They also found that the temperature at the bottom of the hole reached a blistering 356°F (180°C). Too hot to continue, drilling officially halted in 1994.  The project produced enormous amounts of geological data, most of which elucidated how little we know about our planet.