Hydrogen sulfide (or hydrogen sulphide) is the chemical compound with the formula H2S. This colorless, toxic and flammable gas is partially responsible for the foul odor of rotten eggs and flatulence. It often results from the bacterial break down of sulfates in 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 H2S is commonly misattributed to elemental sulfur, which is in fact odorless. Hydrogen sulfide has numerous names, some of which are archaic.