New York City is barely above sea level, and global warming has put the city at risk of devastating floods as hurricanes become more common. Today, almost half a million New Yorkers live in flood zones. Although other cities have more land area in their floodplains, New York’s density makes it the American city with the most people living in a flood zone. To save the city, engineers are dreaming up amazing technologies, including a giant deployable wall that folds out around Manhattan, oyster beds in the bay, and waterproof skyscrapers.