A bog is a wetland built of peat, the waterlogged, half-rotted remains of plants, usually sphagnum moss, that gets almost all its water from rain and snow rather than from rivers or groundwater. That makes bog water acidic and very poor in nutrients, so dead plants decay more slowly than new ones grow, and the peat thickens at roughly 1 mm a year; ground counts as peatland once the peat is at least 30 cm deep, and northern peatlands average 1.5 to 2.3 m. Most of today's bogs began forming about 12,000 years ago, in hollows and on flat ground left as the ice-age glaciers retreated. Well-known examples are Clara Bog, a raised bog in County Offaly, Ireland, protected within an 836-hectare conservation area; the Flow Country of northern Scotland, about 4,000 km² of blanket bog made a UNESCO World Heritage Site in 2024; and the Vasyugan Mire in western Siberia, about 53,000 km², the largest peatland in the world. A bog is easy to confuse with a fen, a marsh or a swamp, and the differences come down to where the water comes from, what grows in it and whether peat builds up.
Bog or Fen: Rainwater at Clara Bog, Groundwater in the English Fens
Bogs and fens are both mires, or peat-forming wetlands, and from a distance both look like flat, wet, brownish moorland. What separates them is plumbing. A fen is fed by groundwater or streams that have run over rock and soil and picked up dissolved calcium, magnesium and carbonate. Those minerals buffer the water, so fens range from slightly acidic to slightly alkaline and support a rich mix of sedges, grasses and flowering plants. A bog is ombrotrophic, meaning "fed by rain". Rain carries almost no minerals, so nothing neutralises the natural acidity, and only specialists cope with the result.
Clara Bog in County Offaly shows how the two connect. It formed in a shallow lake basin at the end of the last ice age, after an esker, a ridge of glacial gravel, blocked drainage to the north. Raised bogs like this one typically begin as fens. As peat piles up, the surface rises beyond the reach of mineral-rich groundwater, the plants come to depend on rain alone, and the wetland grows more acidic until it has become a bog: a gentle dome of peat standing above the surrounding land. Clara still shows that dome, although a road built across it in the late 18th century and the drains that came with it split it into an eastern and a western dome and let parts subside by up to 6 m. Today about 460 hectares are a state nature reserve inside an 836-hectare Special Area of Conservation.
The Fens of eastern England are the classic contrast. Covering nearly 3,900 km² inland of the Wash, across south-east Lincolnshire, most of Cambridgeshire and edges of Norfolk and Suffolk, the region is naturally marshy, with lakes or meres that were once permanently flooded. The local word "fen" came to describe the typical marsh there, with neutral or alkaline water and plenty of dissolved minerals. Most of it was drained centuries ago for farmland.
What Sphagnum Moss Does to a Bog
The plant that turns a wet hollow into a bog is sphagnum, a genus of about 380 accepted species of moss. Its dead cells act as tiny water tanks, and depending on the species a sphagnum plant can hold 16 to 26 times its own dry weight in water. That keeps the ground saturated even between rains, and saturated ground runs short of oxygen, which slows the microbes that would otherwise break down dead plants.
Sphagnum also makes its own surroundings more hostile. It takes up mineral cations such as calcium and magnesium and releases hydrogen ions in exchange, acidifying the water, and phenolic compounds in its cell walls resist decay. The result is peat, and water draining out of a bog is stained brown by tannins dissolved from that peat. Few plants thrive in such conditions, so bogs are dominated by sphagnum, heathers and other ericaceous shrubs, and sedges, along with carnivorous plants such as sundews and pitcher plants that take nutrients from trapped insects rather than from the soil.
Bog, Marsh and Swamp: From the Everglades to the Okefenokee
A marsh is a wetland dominated by soft-stemmed plants such as grasses, rushes and reeds rather than by woody plants, usually at the edges of lakes, rivers or the sea. Marsh water tends to be neutral to alkaline, and unlike mires, marshes are not defined by deposits of acidic peat. The Everglades of southern Florida are a large example, a region of flooded grassland where water leaving Lake Okeechobee in the wet season spreads into a slow-moving river about 97 km wide and more than 160 km long, flowing south across a limestone shelf to Florida Bay. Marshes are biologically productive but hold only about 0.1 percent of the carbon stored on land, a tiny share compared with peatlands.
A swamp is a wetland dominated by trees and shrubs. Its water may be fresh, brackish or salty, and large freshwater swamps line rivers such as the Amazon, the Mississippi and the Congo. Some swamps also build peat, which is where the labels blur. The Okefenokee Swamp on the Georgia–Florida border covers about 177,000 hectares, and much of its floor is peat. It has been accumulating for the past 6,500 years in a basin on the edge of an ancient Atlantic coastal terrace, hemmed in by the sandy Trail Ridge. It is the largest blackwater swamp in North America, and both the Suwannee and St. Marys rivers rise inside it. In much of boreal Canada, meanwhile, "swamp" is everyday speech for what scientists would call a bog, fen or muskeg. See our explainer on what a swamp is for the tree-dominated side of the family.
Raised Bog, Blanket Bog and the Vasyugan Mire
Bogs come in several shapes, and the shape follows the climate. Raised bogs, like Clara, grow as domes over old lake basins in lowlands. The Bog of Allen, a raised bog of about 958 km² between the rivers Liffey and Shannon in central Ireland, stretches into counties Offaly, Meath, Kildare, Laois and Westmeath. Its peat has been cut on an industrial scale by the state company Bord na Móna, carried on kilometres of narrow-gauge railway to power stations and processing plants.
Blanket bogs need a climate so wet and cool that peat can form on gentle slopes as well as in hollows, smothering whole landscapes. The Flow Country of Caithness and Sutherland is the largest blanket bog in Europe at about 4,000 km², dotted with bog pools and home to breeding greenshank, dunlin and golden plover. The biggest peatlands of all lie in the boreal north. The Vasyugan Mire, in the watershed of the Ob and Irtysh rivers across the Novosibirsk, Omsk and Tomsk regions of Russia, has been growing for nearly 10,000 years, yet about 75 percent of its present area became waterlogged less than 500 years ago. It is only one part of the peat bogs of the Western Siberian Lowland, which together cover more than a million square kilometres. Our guide to the largest swamps and wetlands in the world ranks these giants against the river floodplains.
Bogs are scarcer in the Southern Hemisphere, partly because there is less land at the right latitudes. The largest is the Magellanic moorland of southern South America, at some 44,000 km². In the tropics, peat forms under swamp forest instead: a 2014 expedition from Itanga village in the Republic of the Congo found peatland described as "as big as England" stretching into the Democratic Republic of the Congo.
From Tollund Man to Holme Fen: What Peat Keeps and Drainage Destroys
Tollund Man and the Bog of Allen Trackways
The same chemistry that builds peat also preserves whatever falls into it. On 8 May 1950, peat cutters in a bog 12 km west of Silkeborg on Denmark's Jutland peninsula found a body so well kept that they called the police, thinking it was a recent murder. Tollund Man had in fact died between 405 and 384 BC, in the Pre-Roman Iron Age, and had been hanged. His eyelashes, beard stubble, sheepskin cap and the hide noose around his neck survived because the cold, acidic, oxygen-poor peat stopped bacteria. Acid works both ways, though: bog bodies keep their skin, hair and clothing while their bones dissolve.
Peat also keeps a running record of the landscape. Pollen grains trapped layer by layer let scientists reconstruct which plants grew nearby over thousands of years. In the Bog of Allen, industrial milling has uncovered wooden trackways, walkways built from prehistory into the medieval period so that people and animals could cross the peat. Wooden structures and objects, which rarely survive in ordinary soil, are preserved in the peat.
The Holme Fen Posts and the Carbon in Bogs
Drain a peatland and it begins to disappear. In 1850 and 1851 landowners used steam pumps to drain Whittlesey Mere in Huntingdonshire, once the largest body of fresh water in lowland England. To measure what followed, engineer John Lawrence drove an oak pile through the peat into the clay beneath, with its top level with the ground; a cast-iron column, said to have been salvaged from the Great Exhibition of 1851, soon replaced it. As the drained peat dried, shrank and oxidised, the ground fell by more than 4 m. The site of the Holme Fen Posts is now the lowest land in the United Kingdom, at 2.75 m below sea level.
The peat that vanished there went into the air as carbon dioxide, which is why peatlands matter far beyond their size. A 2002 estimate by Joosten and Clarke put peatlands at more than 4 million km², about 3 percent of the world's land and freshwater surface, yet holding a third of the world's soil carbon. Northern peatlands alone are estimated to hold around 415 gigatonnes of carbon. They also supply water: about 43 percent of people in the UK and 68 percent in Ireland get drinking water from peatland catchments. Protecting a bog means keeping it wet, which is why restoration at places like Clara starts by blocking the old drains. For a wetland that mixes peat, cypress and open water, read our guide to the Okefenokee Swamp.