Hydrogen sulfide is an inorganic compound that is a flammable acid gas at standard conditions and has a rotten egg smell in low concentrations.
Hydrogen sulfide is a colorless, highly toxic, flammable gas with a stench characteristic of rotten eggs. It is usually caused by bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers where anaerobic digestion can take place. It is also present in volcanic gases, "natural gas" and some well waters. Hydrogen sulfide has many names, some of which are ancient.