A bog, quagmire or mire is a wetland 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). Bogs receive all or most of their water from precipitation rather than from runoff, groundwater or streams. As a result, bogs are low in the nutrients needed for plant growth, a condition that is enhanced by acid forming peat mosses. Water flowing out of bogs has a characteristic brown color, from dissolved peat tannins. Bogs are very sensitive habitats, of high importance for biodiversity.

There are two primary ways that a bog can develop: bogs can form as sphagnum moss grows over a lake or pond and slowly fills it (terrestrialization), or bogs can form as sphagnum moss blankets dry land and prevents water from leaving the surface (paludification). Over time, many feet of acidic peat deposits build up in bogs of either origin. The unique and demanding physical and chemical characteristics of bogs result in the presence of plant and animal communities that demonstrate many special adaptations to low nutrient levels, waterlogged conditions, and acidic waters, such as carnivorous plants.

Read more about bogs on Wikipedia

Source: US EPA and Wikipedia