The Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km and contains 30 million km of ice. Around 90% of the fresh water on the Earth's surface is held in this ice sheet. If all of it were to melt, it would cause sea levels to rise by 58 metres. The ice sheet first formed in the early Oligocene. It retreated and advanced many times until the Pliocene, when it came to occupy almost all of Antarctica.