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Aug 29 / Dave Schumaker

The Deepest Borehole in the World

Earlier this week, Alexis Madrigal wrote a great piece for Wired on how the Soviet Union drilled the deepest borehole in the world during the 1960′s and 1970′s.

The original goal was soon subsumed by the desire to learn more about how valuable ores formed, so the hopes of the Russian effort eventually landed in the middle-of-nowhere mining region, Pachenga. There, the Soviets drilled the deepest hole in the history of the world, more than 7 miles deep.

At the Kola Institute, pictured, the Russians drilled for more than 15 years to reach a crust depth of 40,226 feet, a record that’s never been broken. But however successful the mission was as an exploration, the geological findings from the site remain murky and obscured by the way they emanated out of the fading Soviet scientific machine.

The (awesome website) Damn Interesting also wrote about this same borehole back in 2006. The article, titled The Deepest Hole, is fairly long and does a great job describing the history of the project and challenges involved.

Inside the project’s 200-foot-tall enclosure resides a unique drilling apparatus. Most deep-drilling rigs use a rotating shaft to bore through the ground– using a series of extensions which are incrementally added as the hole grows deeper– but such a method was unworkable with a hole as deep as Kola was planned to be. To overcome this, the Russian researchers devised a solution where only the drill bit at the end of the shaft was rotated. They accomplished this by forcing the pressurized “drilling mud”– the lubricant pumped down the drill shaft– through the specially-designed drill bit to cause it to spin.

Today, the deepest hole ever created by humankind lies beneath the tower enclosing Kola’s drill. A number of boreholes split from the central branch, but the deepest is designated “SG-3,” a hole about nine inches wide which snakes over 12.262 kilometers (7.5 miles) into the Earth’s crust. The drill spent twenty-four years chewing its way to that depth, until its progress was finally halted in 1994, about 2.7 kilometers (1.7 miles) short of its 15,000-meter goal.

The Soviet’s drilling rig was designed such that core samples would be provided along the entire length of the drill shaft, providing researchers on the surface with an intimate look at the composition of the Earth as the drill ventured further downward. Before the superdeep borehole project was undertaken, practitioners of Geology had reached a number of conclusions regarding the Earth’s deep crust based on observations and seismic data. But as is often the case when humans venture into the unknown, Kola illustrated that certainty from a distance is no certainty at all, and a few scientific theories were left in ruin. One scientist was heard to comment, “Every time we drill a hole we find the unexpected. That’s exciting, but disturbing.”

Another interesting drilling project we’ve talked about before is the San Andreas Fault Observatory at Depth. While only a fraction of the depth of the borehole at the Kola Institute (only 2 miles down, as opposed to 7.5 miles), it has yielded some interesting results.

As for me, the deepest borehole I’ve ever worked on bottomed out at about 150 feet below ground surface. Quite a few orders of magnitude difference!

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