Ask Smithsonian: What’s the Deepest Hole Ever Dug?

The answer to the question, says a Smithsonian researcher, is more about why we dig, than how low you can go

How low can we go? That’s the challenge that has been taken up by scientists around the world over the last five decades, with each seeking to drill a hole that will go deeper than before.

The goal: the Earth’s mantle. The mantle makes up 40 percent or more of the planet, which has a radius of 4,000 miles. The 1,800-mile-thick mantle sits—or more accurately, heaves up and down—just below the Earth’s crust and above the Earth’s core. The crust is just a tiny portion of the planet—averaging three to 25 miles thick.

The mantle is the major engine driving the planet’s constant evolution and contains a geological record of much of the Earth’s history.

“If we have a better knowledge of what the mantle is and how the mantle behaves, we have better knowledge of volcanoes and earthquakes, and better knowledge of how the planet as a whole works,” said Benjamin Andrews, a research geologist and a curator for the National Rock and Ore Collection at the Smithsonian's National Museum of Natural History.

Scientists took their first crack at the mantle in 1958 with Project Mohole. American engineers drilled through the Pacific Ocean floor off Guadalupe, Mexico. But Congress discontinued funding in 1966 before the drillers ever reached the mantle.

The quest to drill deeper created a global scientific contest akin to the Space Race. In 1970, Soviet geologists took on the challenge, setting their drills over the Kola Peninsula, which juts eastward out of the Scandinavian landmass.

The Kola Superdeep Borehole was just 9 inches in diameter, but at 40,230 feet (12,262 meters) reigns as the deepest hole. It took almost 20 years to reach that 7.5-mile depth—only half the distance or less to the mantle. Among the more interesting discoveries: microscopic plankton fossils found at four miles down. The Kola hole was abandoned in 1992 when drillers encountered higher-than-expected temperatures—356 degrees Fahrenheit, not the 212 degrees that had been mapped.

The heat wreaks havoc on equipment. And, the higher the heat, the more liquid the environment, and the harder to maintain the bore, said Andrews. It’s like trying to keep a pit in the center of a pot of hot soup.

In 1990, German scientists began the German Continental Deep Drilling Program in Bavaria. The researchers passed through seismic plates and encountered temperatures as high as 600 degrees F. They managed to get down about six miles before they ran out of funds. But they gleaned new knowledge about seismic activity and the crust’s composition.

Not surprisingly—because the crust is thinner—some deep holes have been bored through the ocean floor. The specialized Japanese drillship Chikyu claims the record for the deepest offshore hole drilled for scientific purposes—about 10,000 feet (almost 2 miles) below the sea floor, according to James F. Allan, program director for the Ocean Drilling Program at the National Science Foundation.

The oil and gas industry also claims some deep holes, on land and offshore. BP’s Deepwater Horizon holds the offshore record. The drilling rig—lost in an explosion in 2010— managed to get some 30,000 feet below the sea, or about 5 miles.

Now, the international team that sponsors the Chikyu is endeavoring to top all previous records. The Integrated Ocean Drilling Program has been in operation since 2003 and is mainly funded by the Japan Ministry of Education, Culture, Sports, Science and Technology and the U.S. National Science Foundation. Also lending support: the European Consortium of Ocean Research Drilling, the People's Republic of China, the Republic of Korea, India, Australia and New Zealand, and the Federal Republic of Brazil.

The effort is expected to take many, if not dozens, of years, and may require $1 billion. Chikyu is capable of carrying up to 6 miles of drill pipes at a time. But the drill bits have a limited lifespan, and high temperatures can deform bits and pipes, not to mention creating a mess out of the bore hole. Temperatures can hit 1,600 F where the crust meets the mantle, and as high as 4,000 degrees at the bottom of the mantle.

Drilling to such massive depths requires mapping and seismology studies, but even with those guides, “occasionally we hit a surprise,” said Andrews.

Ultimately, it’s a journey of discovery. “Part of why you’re drilling is because you want to find out what’s down there,” he said.

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