A bog or mire is a wetlan type that accumulates acidic peat, a deposit of dead plant material – usually mosses, but also lichens in Arctic climates. Bogs occur where the water at the ground surface is acidic, either from acidic ground water, or where water is derived entirely from precipitation, when they are termed ombrotrophic (rain-fed). Water flowing out of bogs has a characteristic brown color, from dissolved peat tannins. Bogs are very sensitive habitats, of high importance for biodiversity.