Find out how we would be able to get anywhere in just under forty five minutes, if it weren't for that pesky molten core at the center of the earth.

Imagine getting anywhere on earth in under an hour, without expending any power. This is the idea behind the gravity train, a mode of transport that has been theoretical for almost four hundred years and will, unfortunately, remain theoretical for another four hundred.

The concept of a gravity train was kicked around by no less a mind than Isaac Newton in the late sixteen hundreds. The concept behind it is simple. Tunnel between two spots on earth. Lay track. Put a train on the track at one end and let it fall into the hole. For the first half of the journey, it would just keep falling, being accelerated by the earth's gravity until it reached the midpoint of the journey. After that, although it wouldn't change direction, it would be climbing away from the center of the earth. Because the gravitational pull would now be coming from the opposite direction, the train would decelerate until it finally, gently, rolled out of the earth at its destination.