Peat is found in the acidic and waterlogged ground of bogs and fens. The anaerobic nature of such environments prevents the total decomposition of the plants that grow there. These are mostly mosses, especially of the genus Sphagnum. A compressed, partially decomposed wodge of organic matter builds up over time – this is peat.
Peat is slow-forming. It takes nature about a century to make five centimetres of it. The younger the peat, the paler it is in colour and the more water it holds. As peat ages, it turns a darker brown and its carbon content increases, constituting an intermediate stage in the formation of coal. This high carbon content makes mature peat a useful fuel. It has been burnt in Europe since at least the time of Ancient Greece. Peat-powered thermal power stations still operate in Ireland, Russia, Finland, Estonia and Rwanda.
Although not itself fertile, peat can store a substantial amount of nutrients and therefore makes an excellent compost. Its low temperature and aseptic nature ensure the partial preservation not just of plants and mosses but of all living organisms. Even humans have been mummified in peat bogs, the 10,000-year-old Koelbjerg Woman of Denmark being the oldest example.