It's a hole that formed along a seam in the bed where the floor and the wheel well meet. This is a support channel underneath where it looks like gunk collected and the rust just found a void or scratch or something and I suppose has been just munching for a while.
I was packing the truck for the weekend and noticed a bubble that seems sorta soft, so took a chance fearing the worst and cut into it. Pulled out what I could and painted the whole thing with rust converting prep and covered it with duct tape. I tapped the rest of the floor and didn't feel anything else, but will start peeling off the bed liner.
Or maybe just buy a new bed from Toyota...
I'll have to investigate more but my first thought is based on how easy it was to peel off what I did that dry ice and trying to take it off in chunks would be most successful. Seems heat and chemicals that soften will just result in a mess.
"Silent enim leges inter arma." -- Cicero