Bogs

Also called mires, bogs are watery areas that accumulate peat, are covered by shrubs and moss, and sometimes feature floating islands of vegetation covering deeper pools. Shallow bogs are difficult terrain for a Medium creature, and deep bogs are greater difficult terrain. If a bog is deep enough that a creature can't reach the bottom, the creature has to Swim. Bogs are also acidic, so particularly extreme or magical bogs can be hazardous terrain.