In 1897, geologist Thomas Jaggar (who would later go on to found the Hawaii Volcano Observatory) led a field party to visit "Death Gulch", a small, steep ravine in a remote hot springs area in what is now Yellowstone National Park. Here, the deaths of numerous bears, elk, rodents and insects had been reported. The group came across eight new bear carcasses during their visit to Death Gulch, and Jaggar surmised that their deaths had been caused by exposure to poisonous levels of carbon dioxide (CO2) and hydrogen sulfide (H2S) gases.

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Over a hundred years later, in 2004, five dead bison were discovered in Yellowstone's Norris Geyser Basin. Strangely, the bison lacked markings on their bodies that would suggest they were attacked by predators. Based on the position of their bodies, it appeared that the animals had died suddenly and as a group. Similar to the events at Death Gulch, park scientists concluded that their deaths were likely caused by buildup of lethal levels of carbon dioxide and hydrogen sulfide gases at the ground level, where the animals were grazing.

The gases emitted from Yellowstone's hydrothermal areas are composed mostly of water vapor, a harmless gas. However, other gases that are poisonous to people and animals when present in certain concentrations are also found within these emissions. Carbon dioxide is a colorless and odorless gas, while hydrogen sulfide is colorless, flammable and has the distinctive rotten-egg smell that many people notice in the geyser basins. Both gases are heavier than air. In most circumstances, wind will dilute carbon dioxide and hydrogen sulfide to low concentrations that do not threaten the health of people and animals. In certain very stable atmospheric conditions, though, these relatively heavy gases can accumulate in low-lying areas and pose a serious hazard. This was likely the case in 2004, when the dead bison were found in Norris Geyser Basin after an unusually cold and still night. These specific circumstances likely allowed gases to build up near the ground surface to concentrations that were lethal to the grazing animals.

Under most weather conditions, visitors will not be bothered by the gas emissions that are an integral part of Yellowstone's fascinating hydrothermal areas. However, as recommended by the National Park Service, people should remain aware of potential gas hazards when exploring these areas and leave immediately if feeling sick. Even though there is no record of any human deaths due to exposure to gases at Yellowstone, the lessons of Death Gulch and the Norris bison provide warnings that should be heeded.