Well, I'm embarrassed
to tell you guys how my situation turned out. No, I did not get sprayed, because there was no skunk. This morning I put a gopher bomb in the hole, and covered it. I gave it awhile to fumigate the hole, and when I took the cover off I finally got a good enough look into it to see that it was only about a foot deep into the dirt.
I had a fiberglass stick,it's about 5/16" in dia. and about 4ft. long that I was poking down the hole with. Anyway, when I pushed it into different places in the backstop,I could feel what have to be some air pockets. So my best guess as to what happened is that the hole appeared when the little bit of dirt that was keeping it hidden eroded away. I know it wasn't from bullets chewin their way through the dirt because it was too high up from where the targets are hung.
I'm glad it wasn't a skunk because they are really good at getting rid of yellow jacket nests that are in the ground. So the critters do serve a purpose.
Thanks again for all the advice, and telling your stories.