Geologic studies have shown that the world’s coal deposits range in age from 2 million to 350 million years old, and formed from ancient plants that thrived under swamp like conditions in humid climates. The plant material eventually became buried, at which point geologic processes transformed the material into sedimentary deposits of coal. Although individual beds of coal can be less than an inch thick, economical deposits generally start at around 6 inches (25 cm), with some reaching over 100 feet thick (30 m). It is estimated that approximately 3 to 10 feet of compacted organic matter is required to produce a foot of coal. Special geologic conditions, however, are required for organic matter in a swamp to be transformed into an economical coal deposit. First, a swamp needs to remain saturated for a considerable period of time. Because saturated conditions help block out free oxygen (O2), the microbial breakdown of dead plant matter is greatly reduced. The abundance of water also promotes lush vegetation growth, which when combined with the slow decay rates leads to thick accumulation of partially decayed plant material. Next, this organic matter must become buried in order for it to be preserved within a sequence of sediment.