Here is an article written by one of my professors, Jeff Mitton, here at CUB. It shows a common snapping turtle in Nebraska, taking a defensive posture at Jeff's approach:
http://www.dailycamera.com/ci_18434437?IADID=Search-www.dailycamera.com-www.dailycamera.com
http://www.dailycamera.com/ci_18434437?IADID=Search-www.dailycamera.com-www.dailycamera.com