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.
'91 Pickup - Imelda / '08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'98 Rhygin Juke SS / '12 Gunnar Rockhound 29 / '16 REEB Dikyelous
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein