Today, I found a hole about 10 inches in diameter and a tunnel in the septic field in my backyard (with a mound of dirt nearby). I’m assuming we’ve got a mole. We are animal lovers and do not kill anything, so we’re inclined to leave it be. However, the area around the hole is soft and gives under my feet, so I’m wondering if the tunneling could result in a small cave-in at some point. Since it’s in our septic field, I figure the tunnel couldn’t go down very far; I peered in and the tunnel seemed to level off. So, is there some reason why we should try to get rid of him?
My first thought was dermatological lesion. 
TEN inches? That ain’t no mole. Mole holes (heh) are maybe 2-3 inches (moles are small.) Ten inches sounds more like you have a rabbit (or a family of rabbits,) or maybe even something larger, like a badger.
If someone missed a left turn at Albuquerque, could they wind up in your yard?
No. Does that whittle down the possibilities?
Groundhog.
You should get rid of whatever is digging into you drain bed or whatever part. I doubt you want the mess and expense of thousands of dollars in repairs in the near future.
P. S. vetbridge: I have several of those, but I’m not about to ask you for advice for removing them.
HarmDisc: Are you saying that if it’s a groundhog (or other animal), it could damage the septic system?
Yes
It’s always fun when a large rodent enters the pipe and your sewage backs up.
Sunscreen then. And avoid chasing cars.
Put some moth balls (actually anti-moth balls, i guess) down in the tunnel. That should cause mole, gopher, badger, groundhog, et. al., to abandon the burrow and find more hospitable digs (get it?). They won’t eat the moth balls so there is no real harm to the animal, but they hate the fumes.
CA
How badly do fumes bother an animal living in a septic bed?
We ran into a park ranger this evening and I asked her about it. She thinks it’s an armadillo. I hadn’t even considered that, even though we know there around. She said it wouldn’t hurt the septic system; they don’t burrow deep, and they just hunker down when they’re not foraging, and eventually move on.
If it is truely moles, but I don’t think that it is based on the other poster’s opinions, I’ve found that removing the grubs (the mole food) makes them move along.
To remove the grubs, go to the local Agway (big box hardware stores also probably have it as well) and buy “Milky Spore.” One treatment is good for 25 or so years, as it’s a spore. The spore eats the grubs, and makes more spores, which then wait around for another grub. Rinse & repeat. I had horrible mole problems when I moved into my house, and discovered this stuff. I haven’t had a single mole trail when the snow clears in the 6 years since I treated my lawn. (1Tbs every 4 feet, in a grid. It cost me about $40 to do the entire lawn, 10,000 sq-ft)
People tell you to kill the grubs and the moles go away. It’s not true. They stay around and eat the earthworms, that are rather abundant in our soil.
YMMV and all that. Worked for me, though I don’t know if I have earthworms, or if the milky spore takes care of them too.