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Five bison have died after being exposed to poison gas in a geyser basin in Yellowstone National Park.

The dead animals were discovered March 10 in the Norris Geyser Basin. They probably had been dead about a week, the National Park Service announced Tuesday.

The two adults, two calves and a yearling, were found "lying on their sides, with their feet perpendicular to their bodies," the announcement said. "The unusual position of the carcasses indicates the bison died very rapidly, as a group."

The bison probably succumbed to a combination of hydrogen sulfide and carbon dioxide emitted by nearby thermal features.

Park scientists, lead by geologist Henry Heasler, surmise the animals were grazing alongside the Gibbon River during an unusually cold and still night about March 1, when a cold front passed through the area.

The weather probably caused the steam and toxic gases to remain close to the ground and concentrate in lethal doses.

Hydrogen sulfide and carbon dioxide can accumulate in low areas when the air is still because they are denser than air.

Some of the nearby vents spewed gas that was more than 200 parts per million hydrogen sulfide, which is commonly known as "rotten egg" gas because of its distinct smell.

Humans can easily detect the gas at levels as low as one part per million and "are able to escape an area well before it reaches a toxic level," the Park Service said.

"The fairly constant wind in the Yellowstone area dilutes and disperses gases so that it would be almost unheard of for a park visitor to be overcome by toxic fumes as the bison were," the Park Service said.

Still, animals sometimes fall to the toxic gases.

There is an area known as Death Gulch in the upper Lamar River Valley where dead animals were found in the 19th century.

In 1899, a geologist found six bears, an elk and several rodents and other small creatures there. Another biologist found seven dead bears there in 1899.

Other researchers have noted the presence of deadly gases in Yellowstone over the years.

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