It takes Scott Kiser only a split second to name the one city in the U.S., and probably the world, that would sustain the most catastrophic damage from a category-5 hurricane. "New Orleans," says Kiser, a tropical-cyclone program manager for the National Weather Service. "Because the city is below sea level—with the Mississippi River on one side and Lake Pontchartrain on the other—it is a hydrologic nightmare." The worst problem, he explains, would be a storm surge, a phenomenon in which high winds stack up huge waves along a hurricane’s leading edge.

For years, scientist Bill McGuire of University College London has been warning the world about Cumbre Vieja, a volcano on the island of La Palma off the African coast. If it erupts, he says, it could force a Manhattan-size rock to splash down into the Atlantic Ocean, kicking up 300-story-high waves that would travel out from the island at jet speeds. Within nine hours, 85-foot-high swells would slam Florida’s Cape Canaveral. Now scientists at the University of Hawaii say that this wild scenario has historical precedence: A similar event unfolded 120,000 years ago in the Pacific.