I've happened upon two large underground tunnels while digging. The first was a foot under the surface and approximately 2" in diameter with perfectly smooth sides. I thought it may have been a pipe that rusted away. But now I've discovered another. It was below a large paver and 1 and 1/4" in diameter--again perfectly smooth and round. There was a bit of mud dried mud at the edges and a small (2") shallow cavity beside the hole. Could this be a frog? I don't have moles or voles. Weird.
I agree with Scott that contacting your local extension service might be your best option. I do know that there are some type of crab in your area that could be the culprit. Take a look here for your extension service.
Thanks for the advice. LSU Ag has been helpful with ID of termites....those dreaded formosans. (Here, it's not a matter of IF you have them it's WHEN you'll get them) I'll give see if they have any ideas. Not worried, just curious.