General Knowledge

The Deepest Hole in the World, And What We’ve Learned From It

Deep in western Russia, the frigid desert contains the remnants of one of the most ambitious scientific experiments ever performed. It’s a ruin now, a wasteland of jagged metal and crumbling concrete. If you search around long enough, you will find a rusted disc, bolted to the earth. So unassuming that you might even try to pick it up. But you won’t be able to. It’s the welded-shut cap of a borehole that plummets more than twelve kilometers into the earth, deeper than the deepest depths of the ocean.

It’s the deepest hole on earth. It’s called the Kola Superdeep Borehole, and its existence has nothing to do with petroleum exploration. Rather, when drilling began in 1970, Soviet scientists hoped to eventually drill down to fifteen thousand meters in order to gain a better understanding of the nature of the Earth’s crust. Because the truth is, we know less about what’s under our feet than what’s on the other side of the solar system.

They drilled on and off for twenty-four years, and though they didn’t quite reach their goal when work came to a halt in 1994, the engineers had reached a record depth: 12,262 meters, a record that still stands today. Two decades later, the Kola Borehole remains a remarkable technological and scientific acheivement. To drill it, engineers devised a new method by which only the drill bit at the end of the shaft was rotated, the lubricant, in this case, pressurized drilling mud, was pumped down through a custom drill bit, allowing it to spin. Instruments had to be invented to take measurements at the bottom of the hole.

What did we learn by drilling a third of the way through the Baltic continental crust? For one, there’s water down there, at depths scientists didn’t believe water could be found. They suspect that the water formed from hydrogen and oxygen that were squeezed out of rock crystals due to crazy high levels of pressure that far down. Unlike groundwater, this water originated from the rock minerals themselves. Never before had this been observed. Also surpising, how about microscopic fossils discovered by Russians at depths of up to 6.7 kilometers?

Researchers catalogued twenty-four species of single-cell plankton microfossils over the course of the project, and they weren’t found in the kinds of deposits we’re used to finding them, like limestone and silica. These were covered by organic carbon and nitrogen compounds, preserved thanks to those high pressures and high temperatures so far below the surface. As for those temperatures, by the time the engineers broke through the twelve kilometer mark, where rock samples were dated at 2.7 billion years old, the heat became a major issue.

Researchers thought the temperature of the rocks would be about 100 degrees Celsius. What they found were temperatures in excess of 180 degrees. It was this heat that caused the drilling to come to a stop. Engineers described the rocks at 12 kilometers as acting more like plastic than rock. Of course, as astonishing as this project was, the Kola Superdeep Borehole only made it through a tiny fraction of the Earth’s layers. 12 kilometers is three times as deep as humans have ever gone, but the eath’s mantle desn’t even begin until about 35 kilometers below the surface.

The mantle then continues for another twenty-eight hundred kilometers; the center of the inner core: more than sixty-three hundred kilometers below the surface. Put another way, this borehole which took 24 years to drill, made it roughly 0.002 percent of the way to the middle of the Earth. It’s a big planet, you guys. Thank you for watching this SciShow Dose, especially to our Subbable subscribers.

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