PLANET EARTH Ice Worlds
Terra incognita（字面意思是隐姓埋名的特拉，表示未知的领域） - the unknown land. At the southernmost extreme of our planet the continent of Antarctica（南极洲） is as large as the United States of America. Ninety percent of all the world's ice is found here. This frozen world is largely deserted until the start of spring. Adelie penguins in a hurry. The clock is ticking. Instead of waiting for the summer melt the new arrivals hasten south over the frozen sea. They have come here to breed but polar summers are so short they must be in position before the thaw（融化） starts.
As the sea ice retreats life can journey farther south. Antarctic waters are so rich that visitors come from far and wide to harvest them. Vast numbers of chinstrap penguins come ashore to breed. No bird will lay their eggs directly onto ice so bare rock is a vital commodity. The best patches are worth the climb. The clifftops are soon stained pink with the droppings of tens of thousands of nesting penguins. Only in a land almost entirely covered in ice could bare rock be reckoned an oasis（本意是绿洲，这里指的是宜人之地）. Some will travel into the heart of the continent to find it.
