OP here. This turned into quite an adventure. I'm laying awake the other night, wondering how I'm going to make sure all the squirrels are out from underneath the shed before we try to close it off, and it hit me: Compressed Air!! So, the next morning, my wife watched the front and left side, while I blasted the hole on the right with all my spanking new 5HP Eaton could give it. Sure enough, two of them came flying out the left side, so I kept at it with murderous, full-auto vengeance. All of a sudden, one comes flying out the little hole I'm laying 120psi air into, bounced off my blow gun and goes flying the other way. I have no idea how he even kept the skin on his face.
We kept blasting all the holes, with no further squirrel activity, so we started to fill the holes with concrete.
Then came 3' of chicken wire around the building, and we got two sides done before stopping to grab something to eat:
When I came back outside, I found another hole:
I wasn't certain it was an exit hole, so I blasted more compressed air into it without result, and figured one of the little buggers had just been left behind, and busted out. So, I finished laying mesh, and waited for the landscapers who are working on another project for us to bring the gravel I'd requested.
When I came out that evening, though, I found that there still were squirrels underneath, and they had dug all three holes out again, but were trapped under the chicken wire. They had tunneled around, but presumably given up and gone back under the shed.
I didn't get a photo, but the next morning I found that they had tunneled further, clear to the back of the shed in one place, and to the adjacent greenhouse in another, and had escaped. By this time we'd had it, and figured we'd fill the holes one more time and, if we found another, they'd get 30 minutes of exhaust from my 80's 4Runner that barely passes emissions.
Fortunately, claustrophobia seems to have done the trick, as we filled the holes again and found no others all day. Back down went the wire mesh, and the landscapers covered it with 3" of gravel. It actually looks pretty good, and I'm calling this battle won.
The war is likely not over, and my NY-bred wife, who's not comfortable around guns, and hates the idea of me killing anything, wants these fargin' bastiges dead, and she doesn't care how, or how messy. Unfortunately, buying even quiet 22 ammunition or an air rifle in the current climate is not easy, so I may just have to go with what I have. We haven't seen any sign of them for two days, though.
Thanks for all your help, suggestions, and hysterical comments!!
Mark