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There is an odd dichotomy in human exploration: While we think nothing of going up — jetting through the skies six miles up, skydiving from the edge of space, or launching humans hundreds or thousands of miles into deep space — going down has always proven rather difficult. To this day, the deepest humankind has ever gone is just 7.6 miles below our feet — or just 0.2% of the distance to the Earth’s core. It’s not that we don’t want to go deeper — and there are huge scientific and commercial gains to be made if we could go deeper — but, try as we might, despite millennia of developing ever more advanced tools and materials, and exploration that has taken spacecraft to the edge of the Solar System, the subterranean depths remain firmly off-limits. Why?


Earth’s crust, cutaway diagramThe very short answer is: It gets very hot very quickly as you journey towards the center of the Earth. In the Earth’s crust — the outermost shell of the planet that reaches down around 30 miles (50 km) — temperatures increase by around 25 Celsius per kilometer of depth (77F every 0.6 miles). This means that, once you’re about 10 kilometers (6 miles) down, temperatures are already as high as 250C (482F). Once you’re through the crust (about 30 miles down) and into the upper mantle, which consists of partially molten rock, you’re talking about temperatures in the range of 650-1200C (1200-2200F). We don’t know exactly, but the Earth’s inner core is theorized to have a temperature of around 6000 Kelvin (5,700C, 10,300F). In case you’re wondering, a lot of this heat is believed to come from the decay of naturally radioactive elements (uranium, thorium, and potassium).

This geothermal temperature gradient is fantastic if you want to tap the Earth for geothermal heating and electricity, but it’s absolutely awful for drilling. In general, drilling — from drilling through your wall to drilling for oil — is constrained by two factors: The length of the drill string, and the temperature of the drill bit. The drill string connects the drill bit to the main drive up on the Earth’s surface, providing both torque (turning power) and drilling fluid (to keep the drill bit cool). The drill bit, which churns through hard rock, gets very hot via friction. Drill bit efficacy can be maintained at higher temperatures by using materials with very high melting points, such as tungsten carbide and diamond, but there obviously an upper limit.

The top side of the Kola Superdeep Borehole in Russia

In the case of the Kola Superdeep Borehole, drilling had to cease when a crust temperature of 180 Celsius (356F) was recorded at a depth of 12,262 meters (7.62 miles). The engineers think they could’ve drilled a bit farther, but they decided to quit while they were ahead since the drill bit would’ve ceased to function at 15,000 meters and a predicted crust temperature of 300C. It had taken them 19 years — from 1970 to 1989 — to drill to 12,262 meters. The Kola Superdeep Borehole is still the deepest artificial hole on Earth.

James Cameron Deepsea Challenger lowered into water on its side for test runs

Don Walsh, Jacques Piccard, and James Cameron all share the title for the deepest ahuman has ever travelled, reaching the bottom of the Mariana Trench (around 11,000 meters or 6.8 miles below sea level). We have a fantastic featured story on the technology employed by Cameron’s submersible, the Deepsea Challenger, to reach such depths. The Mariana Trench is the deepest point in the world’s oceans, though, so humankind won’t be going any deeper any time soon. (The Kola borehole was only a few inches across, and any newer boreholes are extremely unlikely to be wide enough to accommodate a human explorer.)



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