By Christina B. Farnsworth. Steam produced by the earth's magma, an energy source once harnessed by the Romans, helps California keep the lights on. Magma may be an untapped resource in other parts of the country, too. Seventy-two miles north of San Francisco, Calpine owns and operates 19 power generating facilities at The Geysers, creating enough electricity to power 850,000 homes.

The secret is more than just boiling water: Calpine harnesses steam at more than 450 degrees Fahrenheit from the magma to power electricity-generating turbines. To maintain the steam reservoir, Calpine injects reclaimed water.