That warmth can be turned into energy that can power everything from a calculator to a car. Gas, oil, and coal are called fossil fuels because they were created out of fossils millions of years ago. They need to be burned to produce energy and that burning causes air pollution. Between that and cars, it's no wonder our cities have such dirty air. Clean, renewable sources are a better option, but even they aren't trouble free.
Hydroelectric power needs a dam, or some other means of harnessing the water's natural flow.
The huge dams in the American West have nearly killed the Colorado River.