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Hydrogen sulfide (hydrogen sulphide in British English) is the chemical compound with the formula H<sub>2</sub>S. This colorless, toxic and flammable gas is responsible for the foul odor of rotten eggs and flatulence. It often results when bacteria break down organic matter in the absence of oxygen, such as in swamps, and sewers (anaerobic digestion). It also occurs in volcanic gases, natural gas and some well waters. The odor of H<sub>2</sub>S is commonly misattributed to elemental sulfur, which is in fact odorless. Hydrogen sulfide has numerous names (see Table).