Both Australia and Antarctica were once fused together as part of the supercontinent Gondwana, neither being an arid and hot landscape nor a cold desert but instead a vast and lush forest, larger than the Amazon Rainforest, spanning thousands of kilometers in size. The reason Antarctica is no longer temperate and forested is because in contrast to most shorelines which disrupt and disperse ocean currents upon contact with the coastline, the waters around Antarctica encircle it unobstructed, forming the Antarctic Circumpolar Current after the Drake Passage opened 34,000,000 (34 million) years ago. Because there is no land to stop this flow of water, it forces the current to become stronger and deeper over time. The Antarctic Circumpolar Current cut the continent of Antarctica off from the warm waters of the north, a thermal isolation further exacerbated by decreasing carbon dioxide levels over millions of years. In a mere 1,000,000 (1 million) years during a rapid cooling phase, although the full shift took tens of millions of years to complete, Antarctica transitioned from an appearance which resembled Australia during its wetter, forested period of the Cenozoic Era, to become a frozen tundra, virtually uninhabitable by humans until the mid 20th century, due to its subzero temperatures which consistently remain far below any other location upon Earth, as well as being locked in by vast ice sheets which formed 14,000,000 (14 million) years ago