Organic soil or deposit. Peat formation occurs, when decomposition is slow due to anaerobic conditions. The sites where peat forms are fens or bogs. Main peatforming plant in bogs is sphagnum.

Largest peat occurences in Canada and Russia. In Canada, peat is mined for horticultural use. They destroy precious wetlands that took +/- 10000 years to develop to grow flowers in the frontyard. If you use peat for your garden and like to keep some biodiversity on this planet, use compost instead. In some countries, like Finland and Ireland, peat is mined to be used as fuel.