SAN FRANCISCO (Bloomberg) - Google Inc. plans to invest $10.3 million in companies trying to generate power from underground heat sources, the latest attempt by the owner of the world's most popular Internet search engine to promote renewable energy.

Google.org, the company's philanthropic arm, will invest in two geothermal-technology companies, said Dan Reicher, director of climate and energy initiatives. The Mountain View, California-based company also will give $500,000 to the geothermal lab at Southern Methodist University in Dallas.

Geothermal energy comes from naturally occurring heat at depths starting at about two miles (three kilometres) below the earth's surface.

"It's a potentially huge resource," Mr. Reicher said in an interview. "It's something we really need to add to the renewable energy mix in the country and around the world."

The company has said it will fund Google.org, which includes the Google Foundation, with one percent of its stock and one percent of its annual profit.

Today's announcement includes $6.25 million for AltaRock Energy Inc. and $4 million to Potter Drilling Inc., two companies that are pursuing technologies that would reduce the cost of reaching the underground energy sources.

Both companies are exploring "enhanced" geothermal systems, which fracture hot rock underground and circulate water through it to produce electricity in a turbine.

Google created the so-called Renewable Energy Cheaper Than Coal initiative in November with the expectation it will eventually generate revenue from alternative energy.

The program's investments include $10 million in eSolar Inc., a Pasadena, California-based solar power company, and $10 million in Makani Power Inc., based in Alameda, California, which is exploring wind-power generation at high altitudes.

On geothermal energy's potential, Mr. Reicher cited a 2006 Massachusetts Institute of Technology study in which researchers found that two percent of the heat below the continental US between three to 10 kilometres could provide more than 2,500 times the country's total energy use each year.

"There is a real cost curve to come down and we can do it in years and not decades," said Mr. Reicher.