Well, the 'cave' is caused by a spot where 3 layers of metal are pressed together forming a water/moisture/mud trap. If you look at Dave's second pic, there's an inner and outer skin, and a third in the middle that comes from the inside. So when you see it on the inside of the car it's already gone through 3 layers.
Not if you leave the inner layers rusty though, they'll just rot more and more? If you weld it back horizontally though it won't be a trap and will still brace the plates.
Or just become French?
everyone needs to pull that fuel filler cover off and clean behind it, you will cry when you see whats back there
Not buying a nissan then? lol.
If the old area isn't cleaned up fully the new metal will only rust faster than the old stuff. Quite scary how bad it can get. I'm sure if you got in to Rick's boot you could just about turn the rear wheel with your hand.
P.s. hawleeee your car is suhweet
slapping it up into the crack