Picture a hole running straight through the planet, from one side to the other, with you standing at the top. You step off the edge. What happens next?
This question has been bothering curious people for more than 300 years, and the answer is stranger and more elegant than you’d expect. In the idealized version of the problem, you’d fall, speed up, whip through the center at thousands of miles per hour, slow down on the far side, and stop right at the opening on the other end. Then you’d fall back. Without friction, you’d keep doing this forever.
Physicists call the tunnel a gravity tunnel and the vehicle a gravity train. Let’s walk through what would really happen, what the math says, and why we can’t build one.
Where the Idea Came From
The thought experiment is old. In 1679, the English scientist Robert Hooke wrote to Isaac Newton asking what path a falling body would take if it could pass through the Earth. That correspondence is part of the history of how Newton worked out his ideas about gravity. The Wikipedia entry on the gravity train gives a good summary of the early history and the later physics literature if you want to trace it back.
In the 20th century, physicists turned it into a staple of introductory mechanics courses. Paul Cooper’s 1966 paper in the American Journal of Physics, “Through the Earth in Forty Minutes,” is one of the best-known treatments. The title gives away the punchline.
The Simple Version: Gravity Gets Weaker as You Fall
Most people assume gravity gets stronger as you drop toward the center of the Earth, since you’re getting closer to all that mass. It does the opposite.
Here’s the key idea, first proved by Newton. When you’re inside a spherical shell of matter, the pull from the shell cancels out completely. Only the mass beneath you, the part of the planet closer to the center than you are, pulls you inward. As you descend, more and more of the Earth ends up above you, and that mass no longer counts.
If the Earth had the same density everywhere (it doesn’t, but bear with me), the result is clean. Gravity at any depth is proportional to your distance from the center. Halfway down, you’d feel half your normal weight. At the very center, you’d feel nothing at all, because mass surrounds you evenly in every direction and the pulls cancel.
That’s a restoring force that grows with distance from the center, exactly like a spring. And a spring-like force produces a very specific kind of motion: simple harmonic motion, the same math that describes a pendulum or a mass bouncing on a spring.
The Numbers: 42 Minutes Across the Planet
For simple harmonic motion, the time for a full back-and-forth cycle depends only on the force constant and the mass. For a body in a uniform-density Earth, it works out to:
T = 2π × √(R / g)
where R is Earth’s radius (about 6,371 km, per NASA’s Earth fact sheet) and g is surface gravity (9.81 m/s²).
Plug those in and you get a period of about 84.4 minutes. You’d cross to the other side in half of that, roughly 42 minutes.
Your speed would peak at the center at about 7.9 kilometers per second. That’s around 28,000 km/h, or about 17,700 mph. For comparison, a commercial jet cruises at roughly 900 km/h, so you’d be traveling about 30 times faster.
A Strange Coincidence: The Orbit Connection
Here’s a detail I love. That 84-minute figure should sound familiar if you follow spaceflight. It’s almost exactly the orbital period of a satellite skimming just above Earth’s surface, in theory, if the planet had no air and no mountains.
It isn’t a coincidence. The math for a low orbit and the math for the gravity tunnel both come from the same quantity, √(R/g). One trip is a circle around the planet, the other is a line through it. Project the circular orbit onto a straight line and you get the tunnel motion. If you want to explore that connection further, the Wikipedia article on orbital period covers the underlying relationship.
The Real Earth Isn’t Uniform (and It Changes the Answer)
The 42-minute figure comes from pretending Earth has the same density from crust to core. It doesn’t. The core is far denser than the crust, and the shape of that density profile matters.
Geophysicists describe Earth’s interior using the Preliminary Reference Earth Model, a standard density profile built from seismic data. When physicists plug a realistic density profile into the tunnel problem, they get a different, faster answer. Alexander Klotz’s 2015 paper in the American Journal of Physics, “The gravity tunnel in a non-uniform Earth,” found a travel time of about 38 minutes, rather than 42.
The reason is that real gravity doesn’t drop off in a straight line as you descend. Because so much mass is packed into the core, gravity actually increases slightly for the first part of the descent, peaking at around 10.7 m/s² near the boundary between the mantle and the core, about 2,900 km down. You’d feel heavier, not lighter, for part of the trip. After that, gravity falls off sharply toward zero at the center.
The extra pull early on speeds you up, and your top speed rises to nearly 10 km/s. That’s why the trip is shorter than the simple model predicts.
The Surprise: Every Straight Tunnel Takes the Same Time
This is my favorite result, and it still sounds wrong to me every time I think about it.
In the uniform-density model, it doesn’t matter where the tunnel goes. A hole through the center, a shallower chord cutting between two cities a few hundred kilometers apart, a tunnel from Paris to Tokyo: each one takes the same 42 minutes, as long as it’s a straight line and there’s no friction.
A shorter tunnel means a shorter distance, but it also means a weaker pull along the path, because the tunnel isn’t aimed at the center. The two effects cancel perfectly. Short chord, slow trip. Long chord, fast trip. Same total time.
That’s the dream behind the gravity train concept: a vacuum-sealed tube between any two points on Earth, no engine, no fuel, no electricity, just gravity doing the work. In practice, a real tunnel’s travel time would vary a little depending on the actual density of the rock it passes through, and the Earth isn’t uniform, but the principle holds well enough to be tantalizing.
So Why Can’t We Build One?
Well, here are the problems, roughly in order of how quickly they’d kill you.
1. The Heat
The deeper you go, the hotter it gets. Earth’s inner core is estimated to be somewhere around 5,000 to 6,000°C, comparable to the surface of the Sun. Long before you reached the center, any normal material would melt, and you’d be incinerated by radiated heat.
2. The Pressure
The pressure at the center of the Earth is roughly 3.6 million times atmospheric pressure at sea level. No known tunnel wall could hold back that kind of crushing force. Rock at those depths doesn’t behave like solid rock. Under that pressure and temperature, it flows. A tunnel would close up on itself.
3. The Drilling Problem
We can’t even get close. The deepest hole humans have ever drilled is the Kola Superdeep Borehole in Russia, which reached 12,262 meters in 1989. The Kola Superdeep Borehole took roughly two decades to complete, and the drillers had to stop because the rock at the bottom was hotter than expected, around 180°C, and was behaving more like plastic than solid stone.
That depth, 12 kilometers, is about 0.2% of the way to the center. It’s a bit like scratching the skin of an apple and claiming you’ve reached the core.
4. The Air
Even if you solved everything above, you’d still have a tube full of air. A person falling through it would hit terminal velocity, as a skydiver does, and would stop accelerating long before reaching anything close to the speeds in the physics model. The air itself would also get extremely dense with depth, simply from the weight of the column above it.
To make the idealized trip work, you’d need to pump out essentially all the air. That means a sealed vacuum tunnel, which is fine for a train and a disaster for a person in a T-shirt.
5. The Spin of the Earth
This one catches almost everyone off guard. The Earth rotates, so you aren’t falling through a stationary planet. As you fall from the surface, you carry the sideways speed of your starting point with you. At the equator that’s about 1,670 km/h, and it gets smaller as you approach the poles. As you drop toward the center, you’re moving toward a region that’s rotating more slowly, so you drift sideways relative to the walls. This is the Coriolis effect, the same force that bends hurricanes and ocean currents.
In an equatorial tunnel, you’d slam into the side wall early in your fall. The ideal gravity tunnel runs pole to pole along the Earth’s axis, where there’s essentially no sideways velocity to worry about. For any other route, you’d need a clever engineering fix, such as a train on rails or a magnetic track that holds you in line.
Would You Even Land Somewhere Interesting?
Suppose you could dig straight down from your backyard. Where would you come out?
Not where cartoons suggest. Roughly 71% of Earth’s surface is water, and the opposite point on the globe from any land location, called its antipode, is usually ocean. Only about 4% of the planet’s land has land on the opposite side. If you’re in the United States, your tunnel would almost certainly come out in the Indian Ocean. If you’re in Spain or Portugal, you’d pop out near New Zealand, which is one of the few lucky matches. In China, you’d come out in Argentina or Chile.
If you want to check your own antipode, the free tool Antipodes Map lets you click anywhere on a map and see the opposite point on Earth.
Could This Ever Work on Another World?
The same physics applies on any planet or moon, and the travel time depends on the body’s density. The Moon is much less dense than Earth, so a tunnel through it would take longer, about 54 minutes one way, if you assume uniform density. Mars would work out to a trip of roughly 40 minutes.
What’s interesting is that smaller, colder, geologically quiet worlds might be better candidates for tunnels than Earth, because there’s no molten core to melt your hardware. But the engineering remains well beyond anything we can do today.
The Shallower Version That Might Be Real One Day
Here’s the part that isn’t pure science fiction. The chord idea, a straight tunnel between two distant cities, avoids the core entirely. A tube cutting from, say, London to Sydney would be a long, shallow arc, going down maybe a few hundred kilometers at its deepest point rather than thousands. The temperatures are still brutal and the boring work is far outside current ability, but the physics holds up.
The idea resurfaces regularly in engineering circles under the heading of “vactrains” or evacuated tube transport. These designs typically use powered propulsion with magnetic levitation rather than pure free fall, because real-world friction and imperfect vacuum would stop a gravity-only train short of its destination. Still, the gravity-train calculation sets a theoretical baseline: the minimum energy required is essentially zero, and the minimum travel time is under 45 minutes, for any pair of points on the planet.
Whether anyone ever builds something like that is a question of money and materials science, not of physics.
So, What If You Really Jumped In?
Here’s the honest answer, step by step, if somehow you could survive the environment:
- You’d step into the shaft and start accelerating at about 9.8 m/s², the same as any falling object.
- As you went deeper, the pull would first strengthen slightly, then begin to fade.
- After around 19 minutes, you’d reach the center, traveling at close to 10 km/s in the realistic model.
- At that point you’d be weightless, because the Earth’s mass would be pulling on you equally from every direction.
- You’d continue through, now decelerating, and arrive at the opposite opening roughly 38 to 42 minutes after you started.
- If nobody grabbed you at that moment, you’d fall back and repeat the trip forever.
None of this would be survivable in reality. The point of the thought experiment isn’t the tunnel. It’s what the tunnel reveals about gravity: that the force you feel depends on how much mass is below you, that a falling object in a spring-like field has a rhythm independent of how far it travels, and that a trip across the planet could, in principle, be quicker than a TV episode.
Frequently Asked Questions
How long would it take to fall through the Earth?
About 42 minutes in the simple uniform-density model and about 38 minutes using a realistic Earth density profile.
How fast would you be going at the center?
Roughly 7.9 km/s in the simple model, and nearly 10 km/s when you account for the Earth’s dense core.
Would you feel weightless?
Yes, throughout the fall you’d be in free fall, so you’d feel weightless the whole way, though your actual acceleration would change along the path. At the exact center, gravity itself would be close to zero.
Could a tunnel through the Earth ever be built?
Not with any current or foreseeable technology, because of the extreme heat and pressure. Shallower tunnels between distant cities are at least theoretically more plausible.
Does the tunnel have to go through the center?
No. In the ideal model, any straight tunnel between two points on the surface takes about the same time.
Sources and Further Reading
- Gravity train, Wikipedia, overview and history
- NASA Earth Fact Sheet, radius, mass, and gravity data
- Preliminary Reference Earth Model, the standard density profile of the Earth
- Kola Superdeep Borehole, the deepest hole humans have drilled
- P. W. Cooper, “Through the Earth in Forty Minutes,” American Journal of Physics 34, 68 (1966)
- A. R. Klotz, “The gravity tunnel in a non-uniform Earth,” American Journal of Physics 83, 231 (2015)
