Around 23 million years ago, the Drake Passage opened between Antarctica and South America, resulting in the Antarctic Circumpolar Current that completely isolated Antarctica. Models of the changes suggest that decreasing levels of carbon dioxide became more important. The ice began to spread through Antarctica, replacing the forests that had covered the continent. Since about 15 million years ago, Antarctica has been mostly covered with ice. Around six million years ago, the Antarctic ice cap reached the size it is today.