This photo of a rolling arcus cloud was taken on a ship off the coast of Brazil. While it looks menacing enough to fuel another 2012 end-of-the-world conspiracy, it's also downright breathtaking.
These types of clouds are often seen preceding thunderstorms, as the storm's cold downdraft pushes low-hanging warm air in front of it upwards, where it condenses and winds essentially roll into a long tube.
But you don't always need to run for cover when one makes an appearance, since sea breezes have also been known to generate roll clouds, and in the fall they're apparently a common site off the coast of Queensland, Australia. [Nasa EPOD via LiveScience]