When I was a kid, I remember the Memphis Zoo having an alligator snapping turtle from the Mississippi River, that was roughly 1/3 the size of a VW Beetle, and over a century old.
I've come across quite a few in the wild, caught a few fishing, mostly common snappers and sliders, but a handful of alligator snappers, thankfully I've never caught anything near that big.
I did hook something one night, catfishing, I was in a local ~30 year old, municipal reservoir, bottom fishing in 10-15' deep stumpy, dead flooded trees, with frozen shrimp as bait. I hooked something, and fought it for 30-45 min, before my braided line snapped. I don't know what it was, could have been a big ass catfish, but I figured it was a big ass turtle.