Hydrogen sulfide is a chemical compound that is a colorless gas with the characteristic foul odor of rotten eggs. It is heavier than air, very poisonous, corrosive, flammable, and explosive. Hydrogen sulfide often results from the prokaryotic breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers. Hydrogen sulfide also occurs in volcanic gases, natural gas, and in some sources of well water. It is also present in natural halite type rock salts, most notably in Himalayan Black Salt. The human body produces small amounts of it and uses it as a signaling molecule.