In 1904, Italy's Prince Piero Ginori Conti became the first person to use thermal energy from within the earth to turn on the lights -- five of them, to be precise. Now, more than a century after his experiment, 24 countries are using geothermal power. The 10,900 megawatts of capacity installed worldwide generate enough renewable electricity to meet the needs of more than 6 million U.S. homes. Geothermal power has grown at just 3 percent annually over the last decade, but the pace is set to pick up substantially, with close to 9,000 megawatts of new capacity projected for 2015. Some 350 projects are under development in dozens of countries.