If you were able to dig a really deep hole, say several miles down into the earth's crust, you'd notice that the temperature would get hotter and hotter.
Near the bottom of the crust things would get really hot, around 1600 degrees Fahrenheit, which is hot enough to melt rocks.
According to one study, we should be thankful for that heat, because without it, much of the United States would be underwater.
Here's why. Heat makes thing expand, and when something expands it typically becomes more buoyant. Rocks are no exception. If heat from the earth's hot mantle, or the region below the crust, didn't heat things up, the North American continent would sink far below sea level. Denver, the "mile high" city would end up 727 feet underwater. New York, meanwhile, would sink more than a quarter of a mile below sea level.
In fact, the only parts of the United States that wouldn't be completely submerged would be the tips of certain mountain ranges, including the Rockies, the Sierra Nevada, and the Cascades in the Pacific Northwest.
Not that anyone should be worried, though. There's still plenty of heat left over from the planet's birth billions of years ago, and from heat produced by radioactive decay of crystal rocks. Plus, even if the earth's inner regions were to cool, it would take billions of years for continents to sink.