Terrain feature found on grasslands.
Marsh tiles can't be used for anything and limit movement without providing cover during battle, imposing a 15% penalty to units' Combat Strength. In most cases, players should drain marshes in their territory (which clears the marsh from the tile and turns the terrain into basic grassland) as soon as they research Masonry. Dutch players, however, have a unique ability to build the Polder, which grants +2 Food (and later +1 Production and +2 Gold, when they've researched Economics) on a marsh tile without having to clear it. It's highly beneficial for them to keep the marshes so that they can be used later!
Although rich in biodiversity, marshes have little to offer a hungry growing civilization, as they must be drained or otherwise altered before they provide any city bonuses. Note that military units in marshes receive a significant PENALTY when attacked.