Old Faithful geyser in Yellowstone National Park, Wyoming, erupts in scalding hot water and steam on a regular basis and is an iconic example of natural thermal energy. Yellowstone extends into Montana and Idaho, protecting a habitat that supports important plant and animal species, including grizzly bears, gray wolves, American bison and elk.

Geysers, hot springs, mudpots and fumaroles are signs of volcanic activity below. Such renewable energy increasingly is being harnessed across the country to reduce dependence on fossil fuels. (© AP Images)