How Deep is the World’s Deepest Hole?

(Last Updated On: December 10, 2018)

It’s been said that we know less about what’s beneath Earth’s surface than we do about the universe itself. However, the desire to know more about our planet’s inner workings would eventually prompt a world-wide race in the 1960s to see who could drill to the center of the Earth first. The Soviets would lead the way, beginning a mission that would ultimately cost millions of dollars and feats of engineering not seen before. Located in Russia, it was called the Kola Superdeep Borehole, and it reached the deepest point ever before being welded shut in 1994. So, how deep is the world’s deepest hole?

The Opposite of the Space Race

Digging the world’s deepest hole wasn’t about oil or wealth, it was about information and making history. However, doing so would be incredibly difficult to accomplish. Up until that point, everything under the Earth’s crust was only hypothesized.

The race to dig the deepest hole was much like the Space Race which was happening around the same time. The United States was the first country to start drilling, doing so in 1961 with Project Mohole. A drill ship was stationed off the coast of Guadalupe, Mexico, and it drilled five separate holes. The deepest of these reached 601 feet into the sea floor, which was positioned underneath 11,700 feet of water. Ultimately, however, rising costs forced the project to come to a halt. Congress decided that money should instead by allocated towards sending people into space and to pay for the Vietnam War. That’s when the Soviet Union stepped in.

How Deep is the World’s Deepest Hole?

How Deep is the World's Deepest Hole?

In 1970, Soviet engineers began their deepest hole project. They started their drilling in the Kola Peninsula in Northwest Russia. They dug for about 25 years, until they no longer had the funding or the resources to continue.

How deep did they get? The hole itself is 7.5 miles deep (12 km), but interestingly, it is only 9 inches wide in diameter. The Kola Superdeep Borehole is deeper than the Mariana Trench, the deepest point in the ocean. Jokingly, some of the workers on the Kola Superdeep Borehole claimed they could hear the people screaming from Hell the deeper down they got. That’s why it’s sometimes referred to as the “Hole to Hell.”

The goal of the project was to reach the Earth’s mantle, which makes up 84% of Earth’s volume, and has an average thickness of 1,793 miles (2,886 km). While they didn’t make it that far, the project did result in many new discoveries. A number of theories about the Earth’s center were also disproved. Among the discoveries were microscopic plankton found 4 miles down. And another finding was the fossils of about 24 ancient species that were over 2 billion years old!

Why Did the Project End?

The drill the Soviet’s used had been made to withstand temperatures up to 212 degrees Fahrenheit. This worked fine most of the way down. However, the temperatures 7.5 miles below Earth’s surface were much higher than expected (356 degrees Fahrenheit). It was decided that drilling deeper would not be feasible. Today, scientist believe temperatures in Earth’s mantle can reach up to 1,600 degrees Fahrenheit.

Ultimately, the project was abandoned in 1994, and the hole was welded shut with a metal top. If you were to visit the site today, you might accidentally miss the cover, which is surrounded by metal scraps and other debris.

The goal of reaching the mantle is still present, and scientists are continuing to develop new systems to make it that far. Researchers believe that if they can reach the mantle, they’ll discover even more answers to Earth’s history.

What’s in the Center of the Earth, Anyways?

Curious to know what is at the center of the Earth? While we can’t see what’s down there, we do know a few things about it.

Generally speaking, the Earth consists of 4 primary layers:

    • The outer crust
    • The plastic-like mantle
    • The liquid outer core
    • The solid inner core

Earth’s crust is the outermost and thinnest layer of our planet, averaging about 25 miles (40 km) in thickness. The crust is broken up into 15 major tectonic plates that produce geologic activity at their boundaries, like earthquakes and volcanoes.

We already talked about how the mantle is hot and thick. This dense layer of Earth moves like a semi-solid rock, and it’s made up of silicate minerals that are similar to ones found in the crust.

The outer core is made up of mostly liquid iron and nickel, and it reaches temperatures between 7,200 and 9,000 degrees Fahrenheit (4,000 and 5,000 degrees Celsius). It is estimated to be about 1,430 miles (2,300 km) thick.

The inner core reaches temperatures between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius). At 750 miles (1,200 km) thick, the inner core is smaller than our Moon. Scientists continue to make new discoveries about our inner core to this day.

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