A rainforest is a biome, a forested area where the annual rainfall is high. Some mention 1000 mm of rain each year as a limit of what is a rainforest, but that definition is far from complete. Rainforests are primarily found in tropical climates, although there are a few examples of rainforests in temperate regions as well. As well as prodigious rainfall, many rainforests are characterized by a high number of resident species, and a great biodiversity. It is also estimated that rainforests provide up to 40% of the oxygen currently found in the atmosphere.
The undergrowth in a rainforest is restricted by the lack of sunlight at ground level. This makes it possible for humans and other animals to walk through the forest. If the leaf canopy is destroyed or thinned for any reason, the ground beneath is soon colonised by a dense tangled growth of vines, shrubs and small trees called jungle.
Tropical and temperate rainforests have been subjected to heavy logging and agricultural clearance during the 20th century, and the area covered by rainforest around the world is shrinking rapidly. Estimates range from 1 1/2 acres to 2 acres of rainforest disappear each second. Rainforests used to cover 14% of the Earth's surface. This percentage is now down to 6% and it is estimated that the remaining rainforests could disappear within 40 years at this present rate of logging. Further estimates suggest that large numbers of species are being driven extinct, possibly 50,000 a year due to the removal of their habitat.