Yeah, I've seen tank parts like that. You've had a fire in your fuel tank. I know it sounds unlikely, but it happens every once in awhile. There's a spark or something on one of the wires and there is a very quick combustion in the tank. It makes a little pop sound. There isn't much oxygen in a fuel tank so it goes out quickly. It generates just enough heat to deteriorate the corrosion protective coatings on the metal. Then everything starts rusting like crazy. If you take a mirror and a light and look in the tank, you will see that everything above where the fuel level was when the fire happened will be corroded, and everything below it will look fine.
Kurt