Hydrogen sulfide is a colourless gas with a characteristic odour that is soluble in various liquids including water, alcohol, ether, and solutions of amines, alkali carbonates, and bicarbonates. It can undergo a number of oxidation reactions to yield principal products consisting of sulfur dioxide, sulfuric acid, or elemental sulfur. Reaction rates and oxidation products depend on the nature of the oxidizing agent. Hydrogen sulfide is one of the principal compounds involved in the natural cycle of sulfur in the environment. It occurs in volcanic gases and is produced by bacterial action during the decay of both plant and animal protein. It can also be produced by bacteria through the direct reduction of sulfate. Significant concentrations of hydrogen sulfide occur in some natural gas fields and in geothermally active areas.