Why is it hotter deep down in a mine?

Think of stepping into a cave or a cellar on a hot summer day. The air feels cool on your skin, and the stone walls feel cool under your hand. Places just under the ground feel like a shady hideaway from the sun.

Yet miners who travel far down into a deep mine feel something surprising. The farther down they go, the warmer the rock and the air become. So why is it hotter deep down in a mine?

The answer begins far beneath our feet, in the middle of our planet. Deep inside, Earth is hotter than any oven, hot enough to make rock glow. Our planet keeps a huge store of heat tucked away in there.

We live on the outside of all that heat, on a thick shell of rock we call the crust, which means Earth's rocky outer skin. The crust wraps all the way around the hot inside, like a peel around an orange.

Heat always moves from a hotter place toward a cooler place. You feel this when warmth from a mug of cocoa spreads into your cold hands. In the same way, the heat from Earth's inside creeps slowly outward through the crust, toward the cool surface. So what do you think happens to that heat when it reaches the top, up near the grass?

Up near the grass, the heat slips away into the air and the sky. So the rock up there stays cool, just like a cave wall on a summer day.

Deep down, the rock sits closest to the hot middle of our planet. Rock passes heat along very slowly, a bit like a thick blanket, so the warmth builds up down low. That is why the deep rock stays warmer.

Miners dig their tunnels deep down into the crust. With every step deeper, the tunnel reaches warmer rock. The warm rock walls give off their heat, and that heat warms the air around the miners.

There is one more reason for the warmth. Fresh air flows down the long tunnel from the top, and the tall pile of air above squeezes it more and more on the way down. Squeezed air warms up, the way a bicycle pump grows warm while you pump up a tire.

Earth's hot middle sends its heat creeping slowly up through the crust, so the deep rock in a mine sits closer to that heat and stays warmer than the rock near the grass. That warm rock heats the air in the tunnel, and that is why it is hotter deep down in a mine.

Down in the deep tunnel, the miners' lamps shine on warm rock walls. Far above them, the grass stays cool in the summer breeze.