Glaciers are melting. Seas are rising. We already know ocean water will move inland along the Eastern Seaboard, the Gulf of Mexico and coastlines around the world. What scientists are urgently trying to figure out is whether the inundation will be much worse than anticipated—many feet instead of a few. The big question is: Are we entering an era of even faster ice melt? If so, how much and how fast? The answer depends greatly on how the gigantic Thwaites Glacier in West Antarctica responds to human decisions. It will determine whether the stingrays cruising seaside streets are sports cars or stealthy creatures with long, ominous tails.