Swamp is a liquid Material that is sometimes found in the Fungal Caverns. It gives the wet stain just like ordinary water. There are two different variants which are internally called swamp_water and swamp although their name in the UI is identical.
swamp_water is of a murky but translucent green color, and can be made by mixing Toxic Sludge with Mud or either variant of Swamp. On occasion, mixing Toxic Sludge with Water in the proximity of Soil will also generate some Mud and thus also swamp water. When disturbed, Swamp Water tends to form Peat, which is a lighter green liquid that tends to float on the surface of swamp and other water-like liquids. Peat is very flammable, and can sometimes cause the entire body of original liquid to boil into steam if enough Peat has formed.
swamp is an opaque, muddy green-brown liquid. Swamp itself cannot be created, but is instead found in small pockets of biomes that receive the plant-infested modifier, as well as occasionally in the Underground Jungle and Fungal Caverns.