Since peat forms mostly underwater, the carbon in plant material is not broken down by oxygen-dependent bacteria, which normally release carbon dioxide from soils back into the atmosphere. Instead, peat acts like a sponge for carbon. In the tropics, underground peat soils store about ten times as much carbon as the above-ground ecosystems. Collectively, the world’s peatlands are believed to hold about 500 billion metric tons of carbon; Indonesia’s peat swamp forests store about one-sixth of that carbon.