Hydrogen sulfide has a rotten egg smell. Some common names for the gas include sewer gas, stink damp, swamp gas, sour gas, acid gas and manure gas. It occurs naturally in crude petroleum, natural gas, and hot springs and where anaerobic decomposition occurs. Why it is considered so dangerous is that people have a problem detecting the odor over a low odor threshold. Once they lose the smell, they think that H2S has gone away. In fact, it may be increasing in intensity.