Below them, the Florida Straits opened wide. One big cerulean highway thick with the traffic of freighters, towering cruise ships, rusty fishing trawlers, smugglers of drugs and humans, all ferrying their goods back and forth. All expecting smooth, profitable journeys.

It was late September, the tail end of that ominous June-to-November hurricane season during which those who live and work in the tropics and subtropics exist in a state of magical thinking, lulled into a sense of safety by previous years' inactivity. But the waters of the world's oceans are warming, and the dangers are growing. The prime conditions for stronger, stranger storms are becoming more common. On this particular day, as the giant C-130 began its descent, the clear waters of the Bahamas were around 86 degrees, about 2 degrees warmer than usual. It was a small variation that ten days later would have a big impact.

The plane banked hard over the landing strip, scaring off the wild donkeys that stray onto the tarmac. On the ground, the heat enveloped the men like a warm bath as soon as they stepped off the aircraft. As they marched toward the hooches—their four bunk dorms—they passed a newly constructed hangar and a plaque commemorating the old one, destroyed in 2008 by Hurricane Ike. It had taken five years to rebuild this little outpost. But the Coast Guard was determined. A tiny island of about 900 people, Great Inagua sits along the Windward Passage—the strait between Cuba and Haiti—a location that makes it the ideal spot from which to run anti-narcotic-smuggling patrols. That it also puts the Coasties on a well-paved route for hurricanes is a coincidence for which they prepared. The hangar's doors, 28 feet high and over a foot thick, were fashioned from cast concrete; the building could withstand hurricane winds of 180 miles per hour. On an island where nothing much seemed to happen, the Coast Guard nevertheless built a fortress with the expectation that someday something would.

Monday, September 28, 2015, dawned cloudless in South Florida. The night before, forecasters watching the skies from a concrete bunker just outside Miami had seen something interesting: a low-pressure system 400 miles southwest of Bermuda. To the experts at the National Hurricane Center, it looked like the germ of something. Winds were blowing at about 35 miles per hour, but the forecasters didn't seem concerned. “Little change in strength is forecast during the next 48 hours,” one of them wrote at 11 P.M.

Maybe it would turn into a storm, but probably not much more. Major hurricanes don't usually originate in the mid-latitudes. They typically come from the trade winds that blow off the western coast of Africa into the Caribbean, where there is warm water to fuel big tempests. Even if the gathering winds became a storm, projections showed it staying far from land—it looked to the forecasters like it'd curve harmlessly to the north.

Still, Captain Rich Lorenzen, commanding officer back at the Coast Guard's Air Station Clearwater, likes to say that when it comes to weather, as soon as a puff of air wafts off Africa's coast, he's watching. The formation—named Tropical Depression 11—was in his briefing when he came into the office on Monday.