See the the long, dangle-y trail of oil in this NASA photo taken May 17? It's a big problem—and not just for the obvious reason. The oil slick isn't simply spreading here, it's hitching a ride on the Interstate.
The Loop Current is a patch of warm, Caribbean water that pops up into the colder Gulf like a prairie dog sticking its head out of a hole. You can see it above as the orange parabola popping up into the blue. Water follows the path of that arch, up from the Caribbean, back to the Caribbean—and the water that gets back to the tropics can easily join up with the Atlantic Gulf Stream, seen above as the orange arrow rising along the Eastern Seaboard.
That tail of oil in the first picture has almost extended into the orange parabola of the second. So far, NASA says, the Loop Current isn't picking up the oil. But, if it does, the Deepwater Horizon slick could reach the straights of Florida in as little as eight days. And, beyond, the Atlantic.