Will hot rocks yield clean energy?

BASEL, Switzerland — When tremors started cracking walls and bathroom tiles in this Swiss city on the Rhine, the engineers knew they had a problem.

"The glass vases on the shelf rattled, and there was a loud bang," Catherine Wueest, a teashop owner, recalls. "I thought a truck had crashed into the building."

But the 3.4 magnitude tremor on the evening of Dec. 8 was no ordinary act of nature: It had been accidentally triggered by engineers drilling deep into the Earth's crust to tap its inner heat and thus break new ground — literally — in the world's search for new sources of energy.

Basel was wrecked by an earthquake in 1365, and no tremor, man-made or other, is to be taken lightly. After more, slightly smaller tremors followed, Basel authorities told Geopower Basel to put its project on hold.

But the power company hasn't given up. It's in a race with a firm in Australia to be the first to generate power commercially by boiling water on the rocks three miles underground.

On paper, the Basel project looks fairly straightforward: Drill down, shoot cold water into the shaft and bring it up again superheated and capable of generating enough power through a steam turbine to meet the electricity needs of 10,000 households, and heat 2,700 homes.

Scientists say this geothermal energy, clean, quiet and virtually inexhaustible, could fill the world's annual needs 250,000 times over with nearly zero impact on the climate or the environment.

A study released this year by the Massachusetts Institute of Technology said if 40 percent of the heat under the United States could be tapped, it would meet demand 56,000 times over. It said an investment of $800 million to $1 billion could produce more than 100 gigawatts of electricity by 2050, equaling the combined output of all 104 nuclear power plants in the U.S.

"The resource base for geothermal is enormous," professor Jefferson Tester, the study's lead author, told The Associated Press.

But there are drawbacks — not just earthquakes but cost. A so-called hot rock well three miles deep in the United States would cost $7 million to $8 million, according to the MIT study. The average cost of drilling an oil well in the U.S. in 2004 was $1.44 million, according to the U.S. Energy Information Administration.

Also, rocks tapped by drilling would lose their heat after a few decades and new wells would have to be drilled elsewhere.

Bryan Mignone, an energy and climate-change specialist with the Brookings Institution in Washington, D.C., said alternative sources of energy face stiff price competition.

"Currently in the U.S. new technologies in the power sector are competing against coal, which is very cheap," he said.