Hurricane Harvey is whirling towards Texas with winds reaching 130 miles per hour — a Category 4 hurricane that was fueled by an unlucky pit stop over a deep patch of warm water in the Gulf of Mexico.

Warm water feeds hurricanes, which form when a weather disturbance, like a small storm, sucks the moist, warm air over the ocean’s surface into the lower atmosphere. When that moisture-laden air reaches cooler temperatures higher up in the atmosphere, the water condenses to form clouds — which spin and grow, fueled by more warm ocean water as it evaporates.

Warm ocean waters intensify hurricanes because they provide more heat — therefore, more energy — to the storm. And the Gulf of Mexico has been unusually warm this year, the Associated Press reports. But that's not the only reason why Hurricane Harvey has become such a monster storm. Another culprit is an ocean phenomenon called a warm core eddy, says Nick Shay, a professor of meteorology and oceanography at the University of Miami.  

“The combination of warm sea surface temperatures about 2 [degrees Fahrenheit] above average, plus the warm eddy, were responsible for Harvey's intensification,” Jeff Masters, the director of meteorology at The Weather Underground, said in an email to The Verge. (It also helped that the speed and direction of the winds pushing on the cyclone didn’t vary much as the storm climbed into the atmosphere, letting the cyclone stay straight, and strong.)

Warm core eddies form when the edge of a warm ocean current flowing between Cuba and Mexico sheds a ring of warm, deep water into the Gulf of Mexico, Shay explains. These warm rings of water travel slowly from east to west. And when one of them is present in the Gulf during hurricane season, “That’s when we’re in trouble,” Shay says.

That’s because warm core eddies can be 300 to 400 feet deep, so there’s a lot of heat to feed the hurricane. “The deeper warm water plays a big role on intensity change,” Shay says. “Warm eddies are providing a continuous source of heat to the atmosphere.” Masters calls them a “ready-made high-octane energy source” for hurricanes.

On Friday morning, Harvey hovered over one for more than six hours, Weather Underground reports. It was the perfect storm to make Harvey a devastating hurricane.