Smothered pine trees reveal unseen killer.

Nobody could understand why trees were dying in California's Inyo National Forest. When stands of lodgepole pine, hemlock and Western white pine started turning a pallid gray in 1990, rangers first focused on the obvious suspects: disease, pests, and the ongoing drought. But none of these factors could explain the dead zones growing in cancerous patches around Mammoth Mountain, a restless volcano in the eastern Sierra Nevada.

Another curious fact: Since 1990, several people had experienced bouts of dizziness, nausea, and even convulsions after entering small cabins and other confined spaces on the mountain.

Geologist Christopher D. Farrar solved the double mystery by measuring gases in the soil around Mammoth Mountain over the last year. The soil beneath the dead trees contains toxic amounts of carbon dioxide gas, which is seeping out of faults on the side of the volcano, Farrar and his colleagues report in the Aug. 24 Nature.

Carbon dioxide made up an average of 30 to 90 percent of all the gases in soil beneath the 30 hectare region of dead trees, whereas under healthy trees that proportion did not exceed 1.5 percent, says Farrar, a researcher with the U.S. Geological Survey in Carnelian Bay, Calif. By analyzing the ratio of different carbon isotopes, the researchers established that the gas had bubbled out of magma within the volcano.

The elevated carbon dioxide abundance can also explain why people suffered strange symptoms when entering confined cabins, says Farrar. As the gas seeps out of the soil, it gets trapped in any unventilated structure. Farrar measured carbon dioxide concentrations of 25 percent in a small cabin and 89 percent in an underground utility vault--both amounts that could quickly kill a person.

He also measured concentrations above 1 percent in campsite lavatories and small tents--levels far from lethal but still in excess of U.S. health standards for occupational safety. The Forest Service closed a popular campsite near Horseshoe Lake this summer because of the elevated gas levels and also because of the threat posed by standing dead trees.

The campsite will reopen in a few weeks, says Thomas Heller of the Mammoth Ranger Station in Mammoth Lakes, Calif. Mammoth Mountain, a popular ski and summer recreation spot, attracts 3.5 million visitors a year.

The carbon dioxide provides a reminder that hot magma sits beneath Mammoth Mountain, comments volcanologist Stanley N. Williams of Arizona State University in Tempe. The mountain could experience repeats of the small steam explosions that last blasted out craters roughly 500 years ago.

"Seeing a large amount of carbon dioxide coming out in a geologically sudden event gives us a reason to at least start paying more attention to the so-called dormant volcano and make sure there aren't other indications that are suggesting it's ready to go into a new phase of activity," says Farrar.

Volcanologists are also concerned about the potential for a truly gigantic eruption in the area. Mammoth Mountain sits on the edge of a large volcanic crater known as the Long Valley Caldera, which scientists are closely monitoring for signs of unrest.