With composting, you usually want to work with a lot of air and oxygen, aerobic composting. But that is obviously difficult under the water where oxygen is more limited. So what exactly happens when large amounts of bio-matter breaks down in a lake or the sea as opposed to on land and with plenty of oxygen? And are there any man-made composting systems designed to work while fully submerged?
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In an oxygenated body of water, the reactions are pretty similar. Water can dissolve plenty of oxygen, which is why fish (which breathe oxygen just like you do, albeit with more efficient means of absorbing it) can live in it.
If there’s no oxygen, though, decomposition is very, *very* slow, taking centuries to millennia. This turns out to be pretty important to the Earth’s carbon cycle. Plants that grow in (anoxic = without oxygen and acidic) peat bogs end up sinking to the bottom and effectively never decaying. The resulting material, peat, is [one of the world’s largest carbon stores](https://en.wikipedia.org/wiki/Peat), even though it’s mostly found in tundra bogs near the poles and only covers a small part of the Earth’s surface.
Animals and humans who die in bogs can be preserved for ages ([this man](https://en.wikipedia.org/wiki/Bog_body#/media/File:Tollundmannen.jpg), for example, is still quite recognizable even though he died 2,500 years ago!), providing a very valuable source of information for anthropologists studying the history of ancient human civilizations.
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