Aggravating matters, South Florida’s cities, in particular its largest, Miami, are built on porous limestone that’s effectively a rocky Swiss cheese. As the limestone soaks up rising seawater, Miami’s infamous “king tides” — fresh water forced up from drains and pipes by underlying salty water — become more frequent. Because of this limestone geology, Miami and other cities in South Florida cannot build substantial sea walls to hold back storm surges the way other cities, like New Orleans and Rotterdam, have. The water would simply flow underneath.

Yet despite these dangers, South Florida continues to grow. The state as a whole has gained more than two million residents since Hurricane Wilma in 2005. None of these arrivals have firsthand experience of Florida’s unique exposure to natural disasters. Although the state has an exemplary emergency response system, much will depend on how ordinary people react in the face of the hurricane. With a storm the magnitude of Hurricane Irma, people helping other people are the most important resource.

And Florida’s real estate market is booming. Although the 2008 financial crisis plunged millions of homeowners into foreclosure, the property market quickly rebounded. New luxury condos have popped up in Florida in recent years, significant numbers of them in areas likely to flood as climate change causes increasingly ferocious storms. For decades, state and local governments have permitted development in hurricane-prone areas. They have argued that this development adds to the tax base, which can in turn generate funds that can be used to shore up disaster defenses. But such policies ultimately place more property — and people — in harm’s way.

What will happen when all that property is deluged by a megastorm, whether it is Irma or the hurricane after it? In six of Florida’s most vulnerable cities, more than a million homes at risk for storm surge damage are not covered by flood insurance, according to recent analysis. These homeowners face financial ruin. Meanwhile, nearly 1.8 million Florida homes and businesses are in the beleaguered National Flood Insurance Program, which was $25 billion dollars in debt even before Hurricane Harvey; Irma could push it much deeper underwater.