Muskeg is a soil type (also a peatland or wetland type called a bog) common in arctic and boreal areas. Muskeg itself consists of dead plants in various states of decomposition (i.e., peat), ranging from fairly intact sphagnum moss, to sedge peat, to highly decomposed muck. Pieces of wood such as buried tree branches can make up 5 to 15 percent of the peat soil. Muskeg tends to have a water table very near the surface. As well, the sphagnum moss forming it can hold 15 to 30 times its own weight in water, allowing the spongy wet muskeg to form even on sloping ground.

Muskeg is wet, acidic and relatively infertile, preventing large trees from growing, though stunted pines may be found.

Muskegs need two conditions to develop: abundant rain and cool summers. A dead plant that falls on dry soil is attacked by bacteria and fungi and quickly rots. If that plant lands in water or on saturated soil, though, it faces a different fate. Air can't get to it, so the bacteria and fungi can't function well. The cool temperatures slow them down even more. All this slows decomposition, and the plant debris accumulates to form peat and, eventually, a muskeg. Depending on the underlying topology of the land, muskeg can reach depths of 30 m (100 feet) or more.

Muskeg can be a significant impediment to transportation. During the 1870s muskeg in Northern Ontario was reported to have swallowed a railroad engine whole when a track was laid on muskeg instead of clearing down to bedrock. Many other instances have been reported of heavy construction equipment vanishing into muskeg in the spring as the frozen muskeg it was parked on during winter thawed.

Superficially, muskeg resembles a grassy plain covered with short grasses, with strands of trees growing where land protrudes above the water table.

Travelling through muskeg is a very strange experience to those not used to it. It can be very dangerous and arduous. Muskeg can grow directly on top of bodies of water, especially small lakes. Thinner patches can allow animals to drop through, becoming trapped under the muskeg and drowning. Muskeg patches are ideal habitats for beavers, Indian pitcher plants, agaric mushrooms and a variety of other organisms.