Science Questions with Surprising Answers
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Dr. Christopher S. Baird

What would happen if you fell into a hole that went through the center of the earth?

Category: Earth Science      Published: October 4, 2013

diagram of core of earth
Public Domain Image, source: Christopher S. Baird.

For the sake of the argument, let's assume you could dig a wide, perfectly straight hole directly down to the center of earth and out to the other side of earth. Further suppose the hole has a strong enough wall to keep it from collapsing or melting. What would happen if you fell into this hole?

After falling about 0.15 kilometers (0.002% of the way to earth's center), you encounter about 20 atmospheres of air pressure and die from hyperoxia. The farther your corpse falls, the more it becomes crushed by the intense air pressure. Note that this pressure is from the air itself above you pressing down and not from the rocks. As you can see, you do not make it very far at all in your journey through the earth before dying. Let's assume you are able to put on some kind of futuristic pressure suit that can protect you from any amount of pressure. What would happen to you if you fell in the hole with this pressure suit on?

After falling about 1.1 kilometers (0.02% of the way to earth's center), you encounter a temperature of about 320 Kelvin and die of heat stroke. After falling a total distance of about 2.7 kilometers (0.04% of the way to earth's center), your corpse encounters a temperature of about 400 Kelvin and your bodily fluids begin to boil away. Upon falling to about 200 kilometers deep (3% of the way to earth's center), your dried up bones and remnants of flesh encounter a temperature of about 1200 Kelvin and are completely incinerated into dust. Your dust remains then fall the rest of the 6200 kilometers (97% of the way) to the earth's center. As should be obvious, going deep into the earth is generally a bad idea. Even with a pressure suit, you don't get very far before being incinerated from the heat. Note that you don't have to be touching the tunnel walls to feel this heat. The air itself in the tunnel is at these temperatures.

Let's assume you put on some kind of futuristic suit that protects you from all pressure effects, all heat effects, all toxic gas effects, and all radiation effects. Then what happens when you jump in the hole? You accelerate as you fall, picking up speed because of earth's gravity. After about ten seconds, having traveled 0.5 kilometers down (0.008% of the way to earth's center), you reach a maximum speed of about 200 kilometers per hour (120 mph). The air resistance at this speed is high enough to prevent you from accelerating more. The farther you fall, the weaker the gravity becomes because more and more of the earth's mass is above you canceling the gravity from the other side of the earth. Also, as you fall, the air pressure goes up so that the air exerts more force against your motion. With gravity getting weaker, and air resistance getting stronger, your speed steadily drops.

After about a week or so of falling (with a maximum speed of 200 km/h, it takes you a while to travel the 6400 km to the center), you finally reach the exact center of the earth. The strength of gravity at the center of earth is zero because there are equal amounts of matter in all directions, all exerting an equal gravitational pull. Also, the air in the hole is so dense at this point that it is like traveling through soup. The small amount of momentum you have at this point will cause you to overshoot the center of the earth and keep on moving through the hole. But once you are past the center of the earth, "down" is now in the other direction, so that you slow down and reverse direction before getting much beyond the center. You continually fall back to the center of earth, overshoot it under your own momentum, and then fall back from the other direction. This motion is much like a yo-yo or a child on a playground swing who is continually overshooting the lowest point. With such thick air, you eventually lose momentum and stop your yo-yo motion about the center of the earth. You end up stuck floating at the center of the earth.

What if the tunnel was completely evacuated of all its air? Without air, there would be no air resistance. You would therefore accelerate to incredible speeds as you fall, reaching a maximum speed on the order of tens of thousands of kilometers per hour . You reach earth's center in a matter of minutes or hours instead of weeks. With such immense speed, you completely overshoot earth's center. As you travel through the far end of the hole, gravity is now in the opposite direction and slows you down. You are slowed down to zero speed just as you emerge from the hole on the other side of the world. This makes sense from an energy conservation viewpoint: you started at rest at one side of the world, so you must end up at rest on the other side of the world if no energy is lost to air resistance.

Note that numbers in this explanation are very rough estimates and are meant to only illustrate the orders of magnitude to be expected.

Topics: free fall, gravity, high pressure, high temperature, hole, hole through earth, temperature, tunnel