Mires are just one of a whole family of wetland types that encircle the Earth at our northern latitudes or, to a lesser extent, nearer the equator. These include sea-fed swamps like the Everglades.
About 95-98% of peat bogs are water. This is largely thanks to one of the principle plants responsible for the formation of peat bog, sphagnum moss. Remarkably this plant is highly porous, its cellular construction soaking up a far greater mass of water than its body mass. Even more remarkably sphagnum moss grows and decays simultaneously. The growing part is above the water's surface and the decay sets in a little below the surface. This means that there is no "clumping" or root ball development, so sphagnums can grow in very close proximity to one another and form durable floating mats of vegetation.
Underneath, in anaerobic (airless) conditions, the decaying matter sinks through the water to form a bed of peat. The rate of this paludification (yes the experts have a word even for this process) is approximately 1mm a year. Peat soils have a complex chemical nature which has yet to be fully understood.
Courtesy of Geoff Holman