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How long would it take to fall through the Earth?

Jump into a frictionless tunnel through the center of the planet and you’d pop out on the other side about 38 minutes later. Here’s why, and why the answer is 42 minutes for a planet like a bowling ball.

Imagine a tunnel straight through the middle of the Earth, with all the air pumped out and walls so smooth that nothing slows you down. You step off the edge. What happens next?

Gravity pulls you down, faster and faster, until you rocket through the center of the planet. Then gravity starts pulling you back, slowing you down on the climb up the far side, until you drift to a stop just as you reach the opposite surface. If nobody grabs you, you fall back in and repeat the trip forever.

Physicists call this a gravity tunnel (or gravity train). The idea goes back to the 1600s, when Robert Hooke described it in a letter to Isaac Newton. The question everyone asks is: how long does the trip take?

The classic answer: 42 minutes

The textbook version pretends Earth has the same density all the way through, like a giant bowling ball. Then something neat happens: the deeper you go, the weaker gravity gets, in exact proportion to your distance from the center. That makes you bounce back and forth like a weight on a spring.

Work it through and the one-way trip takes about 42 minutes and 12 seconds.

Fun fact: 42 minutes is exactly half the time it would take a satellite to circle the Earth while skimming its surface. Falling through the planet and flying around it take the same amount of time.

The real answer: about 38 minutes

Earth isn’t a bowling ball. Its iron core is more than twice as dense as the rocky mantle above it, so a lot of the planet’s mass is packed near the middle.

That changes everything. As you fall through the mantle, gravity barely weakens. It actually gets slightly stronger, peaking at about 10.7 m/s² at the top of the core, nearly 2,900 km down and almost 10% more than you feel at the surface. You keep speeding up hard for much longer, so you get there sooner.

In 2015 the physicist Alexander Klotz worked the problem out with a realistic model of Earth’s density, built from earthquake data. His answer: 38 minutes and 11 seconds, about four minutes faster than the classic answer.

At the center you’d be moving at nearly 10 km per second, about 35,000 km/h, faster than a satellite in orbit. (In the bowling-ball Earth you’d hit a gentler 7.9 km/s.)

Watch Digby make the trip →

The weird part: every tunnel takes the same time (almost)

Now dig a straight tunnel that doesn’t go through the center, say from London to New York. It’s shorter, and it only dips about 600 km below the surface at its deepest point. Surely the fall is quicker?

In a uniform “bowling ball” Earth, no: every straight tunnel takes exactly 42 minutes, no matter how long or short it is. A short tunnel is gently sloped, so you speed up slowly; a long one dives steeply, so you speed up fast. The two effects cancel perfectly.

In the real Earth the answer depends a little on how deep the tunnel goes, sliding from about 42 minutes for shallow tunnels to about 38 for ones through the core. Here’s what we get with the simplified layered Earth this site uses:

Tunnel Tunnel length Deepest point Fall time
London → Paris 343 km 2 km, still in the crust ≈ 42.2 min
New York → Los Angeles 3,873 km 302 km, upper mantle ≈ 41.9 min
London → New York 5,394 km 599 km, upper mantle ≈ 41.6 min
London → Tokyo 8,687 km 1,710 km, lower mantle ≈ 40.7 min
London → Sydney 12,386 km 4,875 km, outer core ≈ 38.6 min
Madrid → Wellington (nearly opposite) 12,741 km 6,291 km, inner core ≈ 38.4 min

So a 343 km hop from London to Paris and a 12,741 km plunge to New Zealand take nearly the same time.

Pick two places and see how deep your tunnel would go →

Why you can’t actually do it

Even ignoring the problem of digging through thousands of kilometers of rock and molten metal at several thousand degrees, a few things would ruin the ride.

  • Air. A skydiver falling belly-down tops out at about 195 km/h (54 m/s), because air pushes back. At that speed, just reaching the center would take about 33 hours, and the air deep in a tunnel would be squeezed far thicker than at the surface. Without the air pumped out, you’d never make it to the other side: you’d slow down and end up stuck in the middle.
  • Earth’s spin. Unless your tunnel ran from the North Pole to the South Pole, the planet’s rotation would push you sideways into the tunnel wall (the Coriolis effect).
  • Heat and pressure. At the center the pressure is about 3.6 million times the air pressure at the surface. No tunnel wall could survive that.

So it’s a thought experiment, but a fun one, and a real physics problem that people still publish papers about.

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