Amazonia is the world's largest tropical rainforest, spanning more than half of Brazil. Within the 2.5 million square miles of the Amazon Basin resides a wealth of life richer than anyplace else on earth, including 500 mammals, 175 different lizards, 300 other reptile species, tree climbers of every kind, and a third of the world's birds. Millions of species remain undiscovered. Learn about how the rainforest's natural bounty came into existence.