Yes, Old Faithful, the world-famous geyser in Yellowstone National Park, does have a distinct and often strong odor. The smell is primarily characterized by sulfur, which most people compare to "rotten eggs." This scent is the result of hydrogen sulfide gas being released from the geothermal plumbing system deep beneath the earth's surface. As the boiling water and steam are forced through ancient volcanic rock and minerals, the gas is carried to the surface and dispersed into the air during the eruption and the periods of "steaming" in between. The intensity of the smell can vary depending on wind direction and atmospheric pressure; on a still day, the sulfurous "rotten egg" aroma can linger heavily around the boardwalks and the viewing area. While the odor is a natural byproduct of the chemical reactions happening in the Yellowstone Caldera, it can be quite surprising to first-time visitors who expect the mountain air to be purely "pine-scented." However, most visitors find that the visual spectacle of the 140-foot water column easily outweighs the olfactory inconvenience, which is a common characteristic of almost all active hydrothermal features in the park.