The circumference, or distance around the Earth, is approximately 40,075 km, but that depends on where you measure it; around the equator, or from pole to pole. So, to travel overland from one location to its antipode, you’d need to travel 20,037 km. A tunnel, dug from one side of the Earth to the other would be, on average, 12,742 km. So it’s a shorter trip, sure, but that’s not the best part.
If one jumps into the tunnel, they would fall down towards the center of the Earth, accelerating constantly, thanks to gravity. By the time one reached the halfway point, after falling for 21 minutes, they will be traveling at 28,000 kilometers per hour.
Once crossed the halfway point, the velocity would carry you back up the other side of the tunnel for another 21 minutes. This time, however, gravity is slowing you down, so by the time you reach the other end, you come to a perfect stop, just as you arrive at your destination.
If you dig a tunnel between any two points on Earth, you can still take advantage of the Earth’s gravity. Instead of traveling between two antipodes, you could travel a much shorter distance, without piercing so far down.
This concept is called a Gravity Train. For example, you could build a shallow tunnel from London to Paris, that only goes down about 55 kilometers. Evacuate the tunnel, and the gravity train is pulled down for half the journey, and then decelerates naturally for the second half. And amazingly, the journey still only takes 42 minutes. No matter which two points you connect, the journey will only take 42 minutes.
