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Center of the Earth

To the Center of the Earth
A new way to dig really deep

A nagging problem for humans, a species that likes to brag about all the distant planets and moons it has surveyed, is that we've never taken a good look right under our noses. The interior of the Earth is tantalizingly close, by cosmic standards, but how do you get there?
The deepest oil well penetrates a mere six miles (ten kilometers) into the crust (the center of the Earth is about 4,000 miles [6,000 kilometers] deeper). Russian scientists dug the deepest hole on the planet in Siberia, but bottomed out at about 7.5 miles (12 kilometers) below the surface. The Mohole project, a 1950s-era U.S. plan, called for drilling a hole 25 miles (40 kilometers) down to the Mohorovicic discontinuity, the boundary between the hard rocks of the crust and the gooey mantle. Sadly, the only discontinuity Mohole ever encountered involved government funding.
It gets harder and harder to drill deep into the Earth because rocks get softer and softer. Brittle at the surface, rocks become plastic at depth, and the pressure caused by the weight of the overlaying crust—about 52,800 pounds per square inch (3,700 kilograms per square centimeter) at a depth of ten miles (16 kilometers), says drilling consultant William Maurer—collapses deep wells, making further drilling impossible.
What little we know about the interior of the Earth (like the fact that there's a crust, a mantle, and a core, or that there aren't mole people down there) comes from indirect evidence, such as the analysis of earthquakes.
So maybe it's time for a radical new approach to exploring Earth's interior. Caltech planetary scientist David Stevenson says we should forget about drilling holes. Instead, we should open a crack.
Stevenson proposes digging a crack about a half mile (less than one kilometer) long, a yard (about one meter) wide, and a half mile deep (not with a shovel—imagine the back strain) but with an explosion more on the scale of a nuclear bomb. Next, he'd pour a few hundred thousand tons of molten iron into the crack, along with a robotic probe. The iron, denser than the surrounding crust, would migrate downward at about 16 feet (five meters) per second, carrying the probe with it and opening the crack deeper and deeper. The iron blob would drop for about a week and 2,000 miles (3,000 kilometers) to the outer edge of the Earth's core, the probe beaming data to the surface.
Stevenson compares his idea to space exploration. "We're going somewhere we haven't been before," he says. "In all likelihood, there will be surprises."
This proposal can probably be filed in the drawer marked Ain't Gonna Happen. The probe would have to survive temperatures that would melt pretty much anything. It could get sidetracked if the iron started flowing through preexisting cracks in the Earth. And just think of the environmental impact statement. But Stevenson's idea may inspire a new look at an old problem. Great things can come from what seem like crackpot notions.

—Joel Achenbach
    Washington Post staff writer

Web Links

National Academies: Project Mohole
Get a brief history of the Mohole Project and see how it led to deep-ocean drilling still being conducted today.

The Mohole Project
This site describes how the project came to be and why it ended.

U.S. Department of Energy Geothermal Energy Program
Learn about geothermal energy and plans for developing its use more widely.

Geothermal Energy Education Office
Everything you'd want to know about geothermal energy is on this site, from geothermal world statistics and maps to names of experts.

More Articles by Joel Achenbach
Read some of writer Joel Achenbach's columns for the Washington Post.

Free World Map

Atkin, Ross. "Peek Into the Earth." Christian Science Monitor, November 5, 2002. Available online at

Berinstein, Paula. Alternative Energy: Facts, Statistics, and Issues. Oryx Press, 2001.

Gedney, Larry. "The World's Deepest Hole." Alaska Science Forum (July 15, 1985). Available online at

Luhr, James. Smithsonian Institution: Earth. DK Publishing, 2003.

Rosen, Jerome. "Deep Drilling: Probing Beneath the Earth's Surface." Mechanical Engineering (June 1991), 70-76.

Vogel, Shawna. Naked Earth: The New Geophysics. Dutton, 1995.


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