Have aliens invaded? Has this photo been taken on the set of the next big science fiction movie? No. We're back in Nebraska again; a part of the world that sees extreme weather on a regular basis, and where the residents are a little more accustomed to unusual sights in the sky. These clouds are actually the aftermath of a tornado.
The clouds are known as 'mammatus clouds', although some people just go straight on-the-nose and call them 'mammary clouds'; they get their name because they're thought to resemble the udders of a cow. They're an interesting phenomenon to look at from the ground, but in the air they're potentially hazardous to pilots, because their formation creates downdrafts and pouches of warm air. As well as accompanying tornadoes, they've also been observed to appear after volcanic ash is thrown into the atmosphere following an eruption.