Back in the Middle Ages, miners noticed that the deeper they dug into the Earth, the hotter it got. Physicist William Thompson, aka Lord Kelvin of temperature fame, theorized that Earth started out hot and has been cooling ever since. He used the hypothesis to predict the age of our planet, but overshot the number by several million years. What was missing in his theory, was his failure to detect the rigidity of the Earth’s mantle, the layer between the crust and the core. MinuteEarth host Emily Elert explains further.