Surf Solar
So Old I'm Losing Radiation Signs

I'm curious about this. Little is known about its origins ( http://fallout.wikia.com/wiki/The_Cloud ) and I somehow really like this "mysterious" aura of it. It was already there in prewar times and caused destruction, yet it still persists and cloaks the sierra madre skyscraper... What do you think it could be? Just some gas? Or something different? Let's speculate a bit. 
