What's At The Bottom Of The Deepest Hole On Earth?


Robin Andrews

Science & Policy Writer


The now-welded-shut borehole, pictured here in 2012. Rakot12/Wikimedia Commons; CC BY-SA 3.0

There’s something incredibly satisfying about the fact that the appropriately named Kola Superdeep Borehole (KSB) wasn’t created out of a need to find oil. This colossus is only 23 centimeters (9 inches) wide, but it’s 12,262 meters (40,230 feet) deep, easily making it the deepest artificial point on planet Earth – and it came about thanks to a scientific duel.

As explained by Atlas Obscura, American and Soviet engineers were trying to outdo each other in the subterranean realm just as they were trying to beat each other into orbit and to the Moon. America may have won the race to the Moon, but their 1958-initiated “Project Mohole” off the Pacific coast of Mexico lost funding and stopped drilling in 1966 – but the Russians, from 1970 to the early 1990s, kept on going.


The result was the KSB, which consisted of several boreholes attached to a single original hole. The deepest is termed SG-3, and extends a decent way through the continental crust of the Kola Peninsula.

If you’re having difficulty visualizing how deep it is, no worries. You could say it’s 37.8 Eiffel Towers deep. No? Well, alternatively, you could say it’s the same length as 13,045 large adult skunks stacked head to toe.

As you’d expect, plenty of new geological data was obtained through SG-3, but it’s safe to say that the palaeontological revelations took everyone by surprise. The Smithsonian explains that around 6.4 kilometers (roughly 4 miles) down, 2-billion-year-old microscopic plankton fossils were found fairly intact, regardless of the intense environmental conditions down there.

We also found out that a major interpretation of seismic data – that at a certain depth, granitic rock transitioned into basalt – was incorrect, and that slow changes in pressure and temperatures over time were responsible for the phantom geological layer.


It was also discovered that free-flowing water existed down there, squeezed from the rocks and trapped by the incredible pressures.

Similar drilling projects, like Project Mohole and several more recent ones, tend to stop because of a lack of sufficient funds. Kola ceased to be in the early 1990s because the temperatures down there were around an unsustainable 180°C (356°F), not the 100°C (212°F) expected.

This may all sound like a rather impressive feat of engineering, and it is, but the borehole is barely a flesh wound on the Earth itself. With an equatorial radius of 6,378 kilometers (3,963 miles), this vertically ludicrous chasm is a mere 0.19 percent of the way down to the center of the planet.

Can we keep going, though? Can we ever penetrate through to the solid, churning mantle below? Well, that actually depends on where you’re drilling.


Oceanic crust is, on average, no fewer than 7 kilometers (4.3 miles) deep. Continental crust is somewhat less dense, but it’s far thicker, averaging out at around 35 kilometers (22 miles) thick. At those depths, the pressures and temperatures are too great for anything to remain mechanically intact, so why don’t we just drill through oceanic crust?

As it so happens, attempts are being made. One, as noted by Nature, is periodically underway at the Indian Ocean’s Atlantis Bank, where a team are hoping to penetrate through a fortunately cooler segment of the oceanic crust there.

Its high density and the fact it’s underwater does present engineers with considerable problems – and the project has been quite stop-start for the last few years – but that won’t stop them trying to get their hands on a pristine, unaltered piece of the solid, slowly churning mantle beneath.


  • tag
  • geology,

  • pressure,

  • fossils,

  • mantle,

  • drill,

  • borehole,

  • crust,

  • temperatures,

  • kola,

  • superdeep