The last time a major hurricane struck Miami directly, in 1926, it left almost 400 people dead, making it one of the 10 deadliest hurricanes on the record books in the United States. Yet that storm ravaged a sleepy, relatively small resort town of just 100,000. Today, the Miami metropolitan area has more than 6 million residents.

Even as Harvey lingers in the Gulf Coast, dumping rain on an already deluged region, the Atlantic hurricane season continues, and threatens to bring more nasty storms in short order. In the central Atlantic, Irma is some 3,000 miles southeast of Miami Wednesday afternoon, and is expected to become a hurricane later this week.

While it’s impossible to reliably predict where Irma might hit, and what strength it would have when it does, the prospect of another hurricane striking so quickly on the heels of Harvey is daunting. That would especially be true if it turned northward and struck south Florida.

For years, disaster planners have wondered where a major hurricane would strike next. There are plenty of cities that would fare poorly from a hurricane, but even New Orleans—where the loss of life was appalling, and the tag for the damage was the highest of any hurricane since 1900—is a relatively small city. Worse would be a full-force hurricane hitting one of the country’s largest cities, and planners tend to agree that the two cities most susceptible to a catastrophic hurricane are Miami and New York.

The scariest scenario is Miami. While the city is practically synonymous with storms—just ask the University of Miami—it has escaped a direct hit for 91 years, and with it the massive storm surge that might deal irreparable damage.

“It won't survive,” Craig Fugate, the former top emergency manager for both the federal government and the state of Florida, said in 2014.

Any comparison with the 1926 cyclone, a Category 4 storm still known as the Great Miami Hurricane, is only somewhat useful. The growth of the city since then doesn’t just put more people in the path of the storm—it also means that the area presents a greater risk of flooding. As Houston has demonstrated, the more built environment a city has, and especially water-impervious structures like pavement, the less water it can absorb. It’s not just that the city has grown and created more pavement; it’s that just as Houston’s expansion has taken over prairies with remarkable capacity to take in water, Miami’s growth has colonized parts of the Everglades, another water-absorbing system.

And as if that weren’t bad enough, the area is already grappling with regular (not to say normal) floods caused by rising sea levels. Part of Miami and its surrounding cities are commonly inundated by high tides.

The Miami area has seen plenty of bad hurricanes since 1926—especially Andrew in 1992, which caused $26.5 billion in damage. But the National Hurricane Center concluded that Andrew did much less damage that it might have if it had been bigger or struck more directly: “Andrew was a compact system. A little larger system, or one making landfall just a few nautical miles further to the north, would have been catastrophic for heavily populated, highly commercialized, and no less vulnerable areas to the north.”

There are different estimates about what a storm equivalent to the Great Miami Hurricane in power and location would do today, but all of them are horrific. An estimate done by the state of Florida came up with a figure of $120 to $130 billion. A 2011 National Hurricane Center study concluded it would be more like $165 billion, the costliest ever. CoreLogic, a real-estate analytics company, calculated in 2014 that reconstruction value of just the homes in the surge zone would be $103 billion.

While hurricanes are associated far less with New York than with the Southern United States, the Big Apple could become a big disaster if struck directly by a hurricane. It’s the only city in CoreLogic’s study that had more real estate at risk than Miami—much more, to the tune of $251 billion. Sandy, in 2012, showed some of the city’s vulnerabilities, especially infrastructure like the subway and electrical transmission, both of which were badly damaged in that storm.

But Sandy was a relatively weak storm—apparently not even a hurricane, technically, when it made landfall. Despite new awareness of the danger facing the metropolis since Sandy and 2011’s Hurricane Irene, it remains unprepared for a major direct hit. Simply because of the number of people, amount of low-lying real estate, and economic importance of New York City, a direct hit on the city by a strong hurricane might not produce images as apocalyptic as Houston’s, but it could freeze the city for weeks or months and take a bite out of the national economy.

There’s one other city that could make the recent devastation in Houston pale in comparison: Houston. As horrible as the damage from Harvey is—and the scope is still not yet clear—there’s a chance it could have been worse. Storm experts have for years feared a hurricane striking Houston head-on. A hurricane aimed straight at the city could send a 20- to 25-foot storm surge through the Houston Ship Channel, knocking the nation’s second-biggest port out of commission. The surge could slam the many petrochemical plants along the channel, spilling somewhere between 60 and 90 million gallons of oil products into the water, in what could be the nation’s worst environmental disaster in history. Even after the initial recovery began, the economic effects would persist for years, and the health effects for even longer.

That nightmare scenario focused largely on the storm surge, whereas Harvey’s blow to Houston has come not from the sea but from above, in the form of record-breaking rainfall. The hurricane itself made landfall south of Houston, then worked its way north. As a result, it’s tough to compare Harvey directly to a hypothetical direct hit on the city. But a direct hit on Houston is a near certainty, sooner or later: The city averages one major storm every 15 years, and excluding Harvey, the last time that happened was Ike, in 2008. When the storm finally hits, the result might be a replay of this week’s devastation—or something even worse.