A rainforest is an ecosystem that is filled mainly with evergreen trees and that typically receives a large amount of rainfall through most of the year. Rainforests occur on every continent except Antarctica. Tropical rainforests are located near the equator and have high average temperatures and high humidity, while temperate rainforests occur mainly in coastal, mountainous areas in the mid-latitudes.
Rainforests are typically composed of four main layers. The top layer, the emergent layer, contains trees as tall as 200 feet and which are spaced far apart. The upper canopy is a layer of dense vegetation roughly 20 feet thick that houses most of the rainforest's animal species and forms a roof that blocks most light from reaching below. Below this the understory is a low-light layer dominated by shorter plants with broad leaves, such as shrubs, vines, palms and philodendrons. The bottom layer is the forest floor, which is dominated by decaying organic matter and where few plants are able to grow because of the lack of sunlight.
Rainforests are often partially self-watering. That is, plants release water into the atmosphere through transpiration, which helps create the thick cloud cover that typically hangs over the forests and keep them humid and warm even when it is not raining.
The great diversity of endemic species in rainforests is largely a result of the large number of places in which plants are inaccessible to many herbivores and in which animals can hide from predators. These numerous refuges also results in a much higher total biomass than would otherwise be possible. The total number of species residing in rainforests is unknown, although it is possible that there are many millions still undiscovered, particularly in the tropical rainforests.