The Amazon Basin is the world's largest rainforest, drainage area for its second largest river and one of the most biodiverse regions on Earth. Tropical forests have been present in South America for millions of years and were at one point spread over most of the continent. As the Earth cooled and grasslands spread, the rainforests retreated to strongholds like the Amazon Basin, shrinking and growing as ice ages came and went. Today's Amazon rainforest covers around 5.5 million square kilometres and sprawls across nine countries. It is hugely biodiverse, with some 10% of the species known to science calculated to be found there.