The gas hydrogen sulfide (H2S) is poisonous, combustible, and colorless. It smells like rotten eggs. It is created organically, by humans, and from natural sources: H2S is frequently found in natural gas and petroleum deposits and is created when organic matter decomposes. The gas hydrogen sulfide (H2S) is poisonous, combustible, and colorless. It smells like rotten eggs. It is created organically, by humans, and from natural sources: H2S is frequently found in natural gas and petroleum deposits and is created when organic matter decomposes. The gas hydrogen sulfide (H2S) is poisonous, combustible, and colorless. It smells like rotten eggs. It is created organically, by humans, and from natural sources: H2S is frequently found in natural gas and petroleum deposits and is created when organic matter decomposes.