I know you were thinking that your day would be ruined if I didn't post more pictures of the hole.
Here you go:
This evening I put on some already-filthy clothes and got on my belly and lifted some large stones out of the hole. I have now uncovered an opening to a further tunnel down into the earth. Still can't see anything, though, other than more of the landscaping stone bricks, which seem to be holding the whole thing up. I tried to get some good pictures using a flashlight, but I'm not sure how much it helped.