The 5.4 million square-mile Antarctic ice sheet is by far the greatest mass of fresh water on earth. If all it were to melt, it would raise global sea levels some 70 meters (220 feet), driving human civilization inland, and obliterating some nations completely. With human-induced climate change now proceeding rapidly, scientists want to know how rapidly the ice might react. One way to find out: study how the ice has reacted to past warm periods that could be analogous to our own.