PAHOA, HAWAII -- A yellow flag at the entrance to the eight-acre clearing warns that poisonous hydrogen sulfide gas may be in the air.

If there is any, the telltale rotten-eggs aroma is lost in the rain pounding on the gravel road and surrounding forest of ohia and strawberry guava.

At the far end of the clearing stands a 176-foot metal drilling tower. It represents one of the most divisive issues in Hawaii's history: geothermal energy.

Advocates point to the Persian Gulf crisis as fresh evidence of the need for Hawaii to reduce its overwhelming dependence on oil to produce electricity. Tapping pockets of magma-heated water and steam under the east rift zone of Kilauea Volcano is technically feasible and relatively benign on the environment, supporters insist.

But opponents say large-scale geothermal power at Kilauea is unproven, unsafe and unnecessary, if modern energy conservation approaches are followed. Further, they say, spinning a steel-and-concrete web of steam wells, power plants, pipelines, roads and electrical transmission lines along the volcano's central flank will chew up precious acres of native forest.

Not since the Vietnam War have protesters here turned out in such numbers to be taken away in handcuffs. In a nearby National Guard armory, trials have been underway for the last of about 120 adults charged with trespassing during a demonstration in March that drew more than 1,200 to the gate at the drilling site.

The issue has pitted scientist against scientist and generated discord in the state Democratic Party and the cabinet of Gov. John D. Waihee III (D). "The Republicans have abortion, and we have geothermal," Waihee has said.

By far the most oil dependent state, Hawaii burns the fuel for 87 percent of its electricity.

Geothermal proponents, who include an alliance of business and labor organizations on Hawaii's Big Island, advance it as a clean, home-grown alternative to fossil fuels.

Currently used in California, Nevada, Utah and 18 foreign countries, geothermal power plants also are considered an important hedge against "greenhouse" global warming. At most, geothermal generators discharge about 0.01 percent of the carbon dioxide produced by oil- and coal-fired generators, and geothermal waste gases often are reinjected into the ground.

"The increased use of geothermal energy for electrical production, especially for use in rapid transit and electric vehicles, can have an important role in reducing the global dependency on burning fossil fuels," California engineers Wilson and Christine Goddard said in a report to the recent International Symposium on Geothermal Energy in Keauhou.

Some Hawaiians have claimed that geothermal development subverts their worship of the volcano goddess, Pele, an argument that the Supreme Court refused to hear in April 1988.

Other groups, including the Sierra Club, the Natural Resources Defense Council and the San Francisco-based Rainforest Action Network, say geothermal development threatens the nation's last large expanse of lowland tropical rain forest.

"The overriding fallacy in the attempts to justify geothermal based on the oil crisis is that nothing has changed regarding the real issues," said Russell Ruderman of the Big Island Rainforest Action Group. "Geothermal is still unreliable on active volcanoes, is still many times more expensive than efficiency investments, involves land stolen from the Hawaiian people and requires unacceptable environmental sacrifices."

The Wao Kele O Puna Forest is the site of drilling by a partnership of Wyoming companies that wants to provide 25 megawatts of geothermal power to the local utility. Another 25-megawatt project, in papaya fields farther down the slope, was delayed while operators sought permits.

The most ambitious plan, supported by the state and Hawaiian Electric Co. would phase in 500 megawatts of geothermal power here between 1995 and 2007. The electricity would be carried by land line and undersea cable to Honolulu, 200 miles to the northwest.

The cable, crossing the 6,200-foot-deep Alenuihaha Channel, would be the world's longest and deepest electrical transmission line.

The state government and the land owner, Campbell Estate, insist that the project would mean razing only 1 or 2 percent of the 27,000-acre Wao Kele tract, part of the 60,000 acres of native forest in the Puna district. Further, a team of University of Hawaii scientists said, Wao Kele no longer is a pristine example of Hawaiian ohia forest, having been invaded by pigs, mongooses and exotic plants such as strawberry guava.

Other scientists insist that what remains is worth saving, particularly as a source of known or potential herbal medicines.

"My general feeling is none of these areas warrants sacrifice," said Dan Taylor, chief of resource management at Hawaii Volcanoes National Park. "They might be weedy and somewhat degraded, but any forest that gets cleared never comes back."

Simply as a remedy to the greenhouse effect, geothermal power has an overwhelming advantage, said Clint Churchill, Campbell Estate executive director and chairman of the Pro-Geothermal Alliance.

Citing figures from the World Resources Institute, Churchill said replacing oil-generated power with 500 megawatts of geothermal energy has the same positive effect on global warming as planting nearly 500,000 acres of trees.