The Amazon rainforest is the largest tropical rainforest in the world, covering over 5.5 million square kilometers, primarily in Brazil but also in other South American countries. It is home to 10% of the world's known species and 20% of bird species. The rainforest formed 55 million years ago in the Eocene era and has existed for at least that long, though its boundaries have fluctuated with climate changes. It remains threatened by deforestation and climate change impacts such as droughts.