# Could You Dig a Tunnel Right Through the Center of Earth?

Is it possible to drill straight through the planet? AlesVeluscek/Getty Images

Want to really get away from it all? The farthest you can travel from home (and still remain on Earth) is about 7,900 miles (12,700 kilometers) straight down, but you'll have to journey the long way round to get there: 12,450 miles (20,036 kilometers) over land and sea.

Why not take a shortcut, straight down? You can get there in about 42 minutes — that's short enough for a long lunch, assuming you can avoid Mole Men, prehistoric reptiles and underworld denizens en route. Granted, most Americans would end up in the Indian Ocean, but Chileans could dine out on authentic Chinese, and Kiwis could tuck into Spanish tapas for tea [sources: NOVA; Shegelski].

Of course, you'd be in for a rough ride. First, you'd have to pass through 22-44 miles (35-70 kilometers) of continental crust (3-6 miles/5-10 kilometers on the ocean floor) followed by 1,800 miles (2,900 kilometers) of mantle. After that, you'd have to traverse a Mars-sized outer core of liquid iron churning as hot as the sun's surface (10,000 degrees F, or 5,500 degrees C), then a solid, moon-sized inner core, and, some studies suggest, a liquid innermost core [sources: Angier; Locke; NOVA].

For sake of argument (and survival) let's pretend Earth is a cold, uniform, inert ball of rock. While we're at it, let's ignore air resistance.

At Earth's surface, gravity pulls on us at 32 feet (9.8 meters) per second squared. That means that, for each second you fall, you speed up by 32 feet per second — but only near Earth's surface. Gravity is a function of mass, and mass is a property of matter. On the surface, all of Earth's matter lies below your feet but, as you fall, more and more of it surrounds you, exerting its own gravity. These horizontal tugs counterbalance each other and cancel out, but the increasing proportion of mass above your head exerts a growing counterforce to the proportionately decreasing mass below, so your acceleration slows as you near the core. At the planet's center, your acceleration due to gravity is zero — Earth's mass surrounds you, gravity cancels out and you are weightless [sources: Locke; Singh].

You're still moving at a heck of a clip, though, so don't expect to stop there. Halfway to the center, your speed hits 15,000 mph (24,000 kph); 21 minutes after jumping in, you blow past the center at 18,000 mph (29,000 kph). Another 21 minutes later, with gravity slowing you as you go, you reach the far side and stop briefly in midair. Unless someone catches you, you'll then head back the way you came and start all over again. In our idealized case, this will continue indefinitely, like a pendulum or a spring, in a process called harmonic motion [sources: Plait; Shegelski; UCSB].

Of course, reality has a tendency to intrude on even the best thought experiments.

## Reality Intrudes on Planet Earth

Now that we've dived into an idealized ball of rock, let's complicate things a bit with reality.

Under pressure: Boring a tunnel through Earth would require overcoming the mind-boggling pressure exerted by 6.6 sextillion tons of rock pressing inward – that amounts to roughly 3 million times sea level pressure [sources: Locke; Plait; UCSB].

You're gonna carry that weight: A tunnel 25 feet, or 7.6 meters across (slightly smaller than the Chunnel) would displace 20 billion cubic feet (578 million cubic meters) of rock. That's a lot of rock.

It's getting hot in here: The Earth's interior is staggeringly hot due to a number of factors, including kinetic energy from formative impacts, gravitational compression forces, internal friction and radioactive decay [source: Plait]. In fact, the crust alone is hot enough to defeat current tunneling tech: The deepest hole ever dug, the Kola Superdeep Borehole in Russia, reached 40,230 feet (12,262 meters) — only a fraction of the way through the crust — before succumbing to high temperatures. Scientists have bored holes in the ocean floor that reach closer to the mantle, however [sources: Fisher; Levitt; Santoski; UCSB].

Mass effect: Crustal mass variations caused by mountains and sea trenches pale beside the differential densities of Earth's interior layers, which grow denser as you head coreward. Consequently, your acceleration would vary more than we described [sources: Reich; Singh; UCSB].

Fatal attraction: Due to the Coriolis effect and angular momentum, your sideways motion will carry you into a wall before you get terribly far down the shaft.

To understand why, consider a hole drilled at the equator. Whether you stand on Earth's surface or near its core, you complete one revolution every 24 hours, but you don't travel the same distance: at the surface, you travel 24,900 miles (40,000 kilometers), while, halfway to the core, you journey half that distance. You would retain that 1,000 mph (1,600 kph) eastward motion as you fell, while the walls around you would move at an ever-slower eastward rate, causing you to run into them.

To save yourself some rock rash, you could drill from pole to pole, where Coriolis has no effect. However, solar and lunar gravity, which also perturb orbiting satellites, would eventually pull you into the tunnel wall anyway [source: Darling].

Strike a chord: Fun fact: A straight line from any point to any other point through the planet would take the same amount of time to fall through as a tunnel through Earth's center. Although the tunnel would be shorter, gravity would exert less acceleration and the trip would take longer [sources: Plait; Shegelski].

On the plus side, if you wanted to turn the journey into a tourist attraction or a really long subway, the fuel cost would be negligible.

### Author's Note

Politicians like to spout off about how we shouldn't spend money on space when so many problems here on the home planet need solving. I strongly disagree, but perhaps deep exploration of Earth could offer a compromise. To explore the ocean floor, lower crust or mantle requires technology on par with spaceflight — in some ways, it's harder — and could produce comparable economic, educational and technological side benefits.

In many ways, we know more about the moon than we do about our own oceans, or about the Earth's interior. It's high time we changed that.

### Sources

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• Darling, David. "The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity." Wiley. Nov. 8, 2002.
• Fisher, Richard. "The Quest to Drill the World's Deepest Hole." New Scientist. July 5, 2012. (Oct. 5, 2012) http://www.newscientist.com/gallery/deepest-drilling
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• Shegelski, Mark. "Would You Fall All the Way Through a Theoretical Hole in the Earth?" Scientific American. April 21, 2003. (Oct. 5, 2012) http://www.scientificamerican.com/article.cfm?id=would-you-fall-all-the-wa
• Singh, Sunil Kumar. "Gravity." Sep. 26, 2007. (Oct. 3, 2012) http://cnx.org/content/m15087/latest/
• UCSB Science Line. "If I Were to Dig a Hole to the Very Core of the Earth, then Jump, Would I Float Because the Gravity is All Converging on One Spot?" (Oct. 5, 2012) http://scienceline.ucsb.edu/getkey.php?key=3224