Antarctica is almost entirely covered by ice sheets up to two miles (3 km) thick, which contain roughly 70% of the world’s freshwater. The great ice sheets spill out to sea as floating ice-shelves and ice-tongues, which routinely calve massive icebergs into the waters around Antarctica. The Ross Sea ice shelf is one the largest ice shelves in the world. It is the size of France, and though the 200-foot-tall cliffs of ice extend 500 miles, that is just, as they say, the tip of the iceberg. Most of the ice lies hidden below the cold, dark salt water.