Among global warming's most frightening threats is the prediction is that the polar ice-caps will melt, raising sea level so much that coastal cities from New York to Los Angeles to Shanghai will be flooded. Scientists agree that key player in this scenario is the West Antarctic ice sheet, a Brazil-size mass of frozen water that is much as 7,000 feet thick. Unlike floating ice shelves which have little impact on sea level when they break up, the ice sheet is anchored to bedrock well below the sea surface. Surrounded by open ocean, it is also vulnerable, but Antarctic experts disagree strongly on just how unstable it is. Now, new evidence reveals that all or most of the Antarctic ice sheet collapsed at least once during the past 1.3 million years, a period when global temperatures probably were not significantly higher than they are today. And the ice sheet was assumed to have been stable.