The hissing and huffing hillsides of Roaring Mountain in Yellowstone National Park are doing more than just blowing off a little steam.

Scientists measuring mercury levels in the park last month were stunned by what they found near the base of the mountain: probably the highest levels of mercury at an undisturbed natural area that has ever been recorded scientifically.

"I looked at it and did a double take," Mike Abbott, a scientist with Idaho National Engineering and Environmental Laboratory, said Tuesday. "I thought my instrument was busted."

Abbott said several areas between Mammoth and Norris Geyser Basin showed "fairly high" levels of mercury, which is a highly toxic pollutant often associated with volcanic areas.

That information is helping scientists answer a crucial question about Yellowstone: whether the park is an important contributor of mercury in the atmosphere.

The research plays into a larger national issue as the federal government works to regulate mercury emissions from industrial sources such as coal-fired power plants. If places like Yellowstone contribute significant amounts of mercury to the air, one theory goes, then regulations on man-made sources may not be as effective as once thought.

Although man-made mercury emissions are pretty well understood, not much is known about natural emissions.

But after their research this fall, scientists have a better grasp on the role that Yellowstone might play.

"In my mind, it's a potentially big source," Abbott said.

Preliminary estimates from measurements taken in Yellowstone in early September seemed to indicate relatively low levels of mercury. But data collected later in the month, and made public Tuesday, showed otherwise.

Judging by what was measured at Roaring Mountain and other nearby spots in the park, Abbott said it's conceivable — though highly speculative at this point, he emphasized — that Yellowstone Park could emit as much mercury as all the coal-fired power plants in Wyoming.

"That's not a real estimate but something based on just a few measurements," Abbott said. "It could even be bigger than that, we just don't know."

Several places in Yellowstone showed signs of mercury in the air at levels higher than background levels at locations not associated with volcanic activity, mining operations, power plants or other known sources.

But it's the corridor between Mammoth and Norris that has piqued the curiosity of researchers.

Abbott said one possibility seems to be that the higher-than-expected levels of mercury along that stretch might be associated with the acidic sulfate system in that area.

Places like Norris basin, Frying Pan Spring and Roaring Mountain seem to point to a connection between the mercury levels and acid sulfate features.

"We haven't gotten it figured out yet but there seems to be some significant sources there," Abbott said.

At Roaring Mountain, Abbott measured mercury emanating from the clay hillside at up to 2,400 nanograms per square meter per hour, significantly higher than measurements of 200-700 at other sites in the Norris-Mammoth corridor. By comparison, background levels away from geothermal areas range from zero to 10.

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Abbott said he was shocked by the measurements at Roaring Mountain and returned to Yellowstone last week to double-check his figures.

"It knocked me over," he said, adding that he's never seen numbers so high for a natural area that hasn't been mined. "It's one of the highest, if not the highest, ever measured."

Abbott said the mercury from Yellowstone poses no danger to visitors.

But, he said, measurements were only taken at a select number of sites. The unusually high level of mercury raises tantalizing questions about total mercury emissions at Yellowstone, Abbott said.

"Yellowstone is a large area. Now that we know where to look, we'd like to do more detailed measurements to produce a reasonably accurate estimate of total emissions," he said.

Once that happens, federal regulators will have a better idea of how much mercury is emitted by natural sources and how much comes from man-made operations.

In December 2000, the Environmental Protection Agency announced that it would require coal-fired power plants, considered to be the largest source of mercury emissions in the country, to reduce their discharge of the toxic metal.

Mercury from power plants often settles over rivers, lakes and other waterways and can contaminate fish, according to the EPA. When people eat contaminated fish, especially those with high levels of the chemical, they are at a higher risk of neurological and developmental damage, particularly in children and developing fetuses, the agency says.

The amount of mercury in the air has been rising in the last century. About 158 tons of mercury is emitted into the air each year, according to government officials.

No one's sure exactly how mercury moves in the atmosphere and it can be difficult to pinpoint where it comes from without intensive testing.

Until now, no one had tried to quantify Yellowstone's contributions.

Abbott said he's hoping INEEL and other researchers, including the U.S. Geological Survey and several universities, will be able to get funding to take a more comprehensive look at Yellowstone's emissions.

"That's going to be a big job," he said.