what gas smells like rotten eggs

Hydrogen sulfide gas smells like rotten eggs at low concentration levels in the air. It is a colorless, flammable gas that is heavier than air, so it can build up in low-lying areas and enclosed spaces. Hydrogen sulfide is commonly known as sewer gas, stink damp, and manure gas. At high concentration levels, it has a sickening sweet odor. It occurs naturally in some environments, such as wells, and can also be produced by certain bacteria in the groundwater, well, or plumbing system. In addition, hydrogen sulfide gas can result from industrial activities, such as food processing, coke ovens, kraft paper mills, tanneries, and petroleum refineries.