Knarly

[CaraRose]

One big and knarled boy (or girl, I can't tell!). It looks like he was missing an eye.

Anyone good with turtles? Is this a common snapping turtle?

[SARtech]

Interesting! A Wood Turtle perhaps? The top of the shell is confusing me with the yellow and green. hmmmmm.
Here's a link for midwestern turtle info:
http://herpcenter.ipfw.edu/index.htm...Wturtles.htm&2

[CaraRose]

Hrmm... that site says wood turtles don't occur in IL.

The shell's one of the things that's throwing me off as well. His head isn't helping since it's got the big lump on one side.

There's no real size perspective in these, but he was probably 20" from his head to the back of his shell.

[SARtech]

The more I investigate... I think you are right that it is a common snapping turtle. The overall size, the size and shape of the head etc.
The yellowish and green colors on the back is probably just algae/moss etc.