Swamps can be found on some levels. This element consists of three layers: reeds, algae, and water. Pieces can't fall on tiles that have reeds. You can remove the layer by making matches or setting off power-ups next to them. Pieces can fall on to the next two layers, which you can remove by making matches or using power-ups on the tiles.

Remove all of the layers to get rid of the swamp completely. If you don't remove all three layers, then the other layers will regenerate with each move you make. The reeds will grow back and replace the pieces on the tiles.