They are snapping turtles, and if they live long enough, they can get this big. I’ve seen the things big enough to be traffic hazards. I’ve seen city folks wanting to help turtles, but they don’t need or want help- they have a long neck tucked away in there, and can strike like a snake. Larger ones can (and will) take off a human finger.
I will steer clear of any snapping turtles I may happen to come across, for sure!