X2 ..... Not to mention that these sealed rounds would have been full of air where there isn't any powder, now fill the tank up with water and muck, and it becomes elementary physics, those cases collapsed because they just plain couldn't hold the weight of all the other stuff when they were designed to hold atmospheric pressures and not much else. IF there had been any sort of explosion on the inside of the tank, you would expect the interior to be in a MUCH worse shape that it is in ... If there had been an explosion anywhere near the outside, you'd see some bulging, which there is none. I blame that "crack" on poor quality Russians steel made in times of war, and the lack of "modern" metallurgy. As others have stated, it's entirely plausible that this is a stress fracture made by the natural flexing of the steel as the tank was rolled over/dragged out of the mud. Only way to really find out is to take a microscope to it and see how much it oxydised.
I wouldna thunk it'd have been very deep in the mud.
My bet would be on freeze and thaw cycles pounding the crap out of the main gun cases and the hull.
Neat!
Cheers
Trev