Earth is like an onion. The layer we live on, the crust, is like the onion skin. It represents less than 1 percent of the distance between Earth’s surface and its center.

Below the crust is the more malleable mantle, making up nearly half of Earth’s volume. Below that are the metal inner and outer cores.

In the late 1950’s, at the start of the Space Race, the U.S. and Soviet Union began a different contest: to see who could go deeper into the Earth, perhaps even all the way to the mantle.

The U.S. had barely begun when Congress pulled the funding. But the Soviets kept going.

They chose a spot in northern Russia above the Arctic Circle. The Kola Superdeep Borehole project housed scientists and drillers in that remote location from 1970 to 1994.

It took them 20 years to reach 40,000 feet, almost a mile deeper than the deepest ocean trench.

There, the heat was nearly double what they expected, and the rock became plastic, oozing back into the borehole. They tried for a few years to go deeper but could not.

Along the way, they discovered hot mineralized water, new species of microfossils, unexpected rock formations, and an array of gases.

Kola retains the superdeep record, though it only made it one-third of the way through the crust at that location.

We know more about the surface of the Moon than the bottom of the ocean or the inside of our own planet. There’s a lot more to discover right here on Earth!