(FORTUNE Magazine) – A great furnace lies hidden in the depths of the earth. It holds 30 times as much energy as all the earth's oil, gas, and coal deposits -- enough clean, efficient power to last mankind thousands of years. At least that's the belief of advocates of a technique called hot dry rock heat mining, which is slowly moving toward reality. Hot rocks undergird North America and every other continent, albeit at different depths. How hot the rocks are depends on geology; areas of recent mountain building and volcanism are the most promising. In the U.S. that means the West, Hawaii, and Alaska -- but if you're willing to drill deep enough or settle for less than scalding water, even the Northeast qualifies (see map). The biggest step toward cashing in on this gift of nature so far: a four- month nonstop test at Fenton Hill, New Mexico. There, scientists from nearby Los Alamos National Laboratory are in the midst of the most extensive demonstration yet. At a depth of 8,000 feet they hydraulically opened up cracks in a granite reservoir with a volume of 750 million cubic feet -- the size of several superdomes. Two wells reach from the surface to the rocks, which are a torrid 400 degrees F. Through one well, cold water descends to the hot rocks and is forced through the cracks at a pressure 200 times greater than that in the tires of your car. The pumps push the heated water back to the surface through the second well. In a real power plant, the water would heat butane that would expand and drive turbines, generating electricity at a cost the scientists claim would be competitive with conventional electric plants -- at least in the West. David V. Duchane, who leads the project at Los Alamos, foresees the first operating plant going onstream in five years or so. Helping realize his dream is Geolectric Power Corp., a Reno, Nevada, startup that is working with the city of Clearlake, California, to build an operating plant. Clearlake is in the Coast Range geyser country that supplies a good deal of steam for generating San Francisco's electricity. But the geysers seem to be running out of gas, so electricity from hot rocks would be a welcome substitute. Other U.S. companies, such as New Mexico's Plains Electric and San Diego Gas & Electric, are following the Los Alamos work closely and have sent advisers to the project. Interest is stirring around the world; tests are under way in Japan, Britain, France, Germany, and Russia. Although mining hot rocks for heat requires no exotic new equipment or technologies, the overall technique remains in its infancy. It has risks -- hitting porous rock, for example, that could act like a leaky bucket -- but the researchers consider them significantly smaller than the chance of hitting a dry hole in oil and gas exploration, which can run as high 20 to one.