A rainforest is a forest that receives heavy rainfall. The most notable rainforests are in the tropics or subtropics, mostly in the Intertropical Convergence Zone. The biggest rainforest is the Amazon rainforest, which is mostly in Brazil. Forests like this have extraordinary biodiversity. Biologists say over half of all plant and animal species live in the rainforest. Also more than 1/4 of all medicines come from here. Even though they only cover 6% of the Earth's land area they are still an important source of oxygen.