Rainforests are forests with high amounts of rainfall, which cause them to have very different characteristics than other forests, such as coniferous forests. Rainforests are variably defined as having a average annual rainfall greater than 1750 mm and 2000 mm (68 inches to 78 inches). There are two types of rainforest: temperate rain forests, which are found in small amounts on the coasts of all continents except Africa and Antarctica, and tropical rainforest, such as the Amazonian rainforest in South America, possessing the numerous unique flora and fauna synonymous with the term "rainforest."
Tropical rainforests can be found in South America (Amazon Rainforest), Africa (African Rainforest), and Southeast Asia (Southeast Asian Rainforest), Madagascar, and that's all. Just a couple hundred years ago, tropical rainforests covered 12% of the surface area of the continents, but today that number has shrunk to less than 6%, due both to human-caused deforestation and the southward creep of the Sahara Desert into the African Rainforest. The majority of the world's rainforests are located within 20 degrees of the equator, where it is warmest, and often, wettest.
Although the world's rainforests only account for 6% of the surface area, they contain two-thirds of plant and animal biodiversity on the planet. They have also been called "the lungs of the Earth," although this is false, as rainforests do not actually produce more oxygen than they take in. Rainforests are covered in evergreen broadleaf trees, some as tall as 80 m (260 ft).
The rainforest biome has a layered structure. At the top is the emergent layer, where the tallest trees poke through the canopy below. The emergent layer is usually between 45 m (150 ft) and 55 m (180 ft) high, although on occasion, certain very tall trees will project 80 m (260 ft) above the ground. The emergent layer is populated by eagles, butterflies, bats, and certain monkeys. The plants here must be resistant to strong winds and high temperatures.
Below the emergent layer is the famous canopy, the area of greatest biodiversity in the rainforest and on the Earth itself. About 40% of all plant species and more than half of all insect species on Earth are thought to live in the canopy, which only started to be probed by scientists in the 1980s. The canopy is 30 m (100 ft) to 40 m (130 ft) above the ground. Here, the branch is so great that it forms a more or less continuous sheet of foliage over areas many hundreds of thousands of square miles in extent.
At the bottom of the rainforest is the forest floor. Little light reaches here, and nutrients are often washed away by the rain. Numerous bacteria quickly break down organic material, precluding the formation of humus. The forest floor has low biodiversity compared to the canopy above it, but still greater biodiversity than any other habitat on the planet.