It could be any of the above as Dorie has mentioned, I bet it is groundhogs.
Be careful it is not Rats,! they will dig and burrow under sheds, houses, etc. I heard Fox urine which you can buy over the net controls small animals. I have it posted on another site in regard to how well this fox urine works but have had no replies so far. I think I just may go buy some to keep away mice from getting into my garden since I live in a rural area. BTW I am having problems with Carpenter bees right now which I am searching for solutions to fix the problem. They are drilling numerous amounts of holes into my deck. I hope you find was is causing your problem. P.S. Moth balls are also recommended as a deterent of small animals. But the smell is awful.
Good Luck.!
Jannie

In my neck of the woods it would be a rabbit or a possum.

Susie

I had same problem with skunks and groundhogs. Moth balls worked like magic. They vacated immediately.

In some areas of the country it is now illegal to use mothballs although if I don't get rid of the vole that has taken up residence next to my house I may have to resort to mothballs if the traps and the poison don't get rid of it.
Penny

I'm pretty sure there is a family of raccoons under mine

In every area of the country it is illegal to use moth balls except as prescribed on the package. Large print on the side says it is illegal to use this product except as prescribed. Now I doubt that the moth ball police will come and arrest you if you do use them to deter animals but I also doubt that you really want the product in your soil, even under a shed.

Hey Gottagarden, where did you put the Mothballs? I have a ground hog that visits my yard daily. The hole is just over my property line on my neighbors side.

We have groundhogs in the Chicago area. We put basketballs over the tunnel holes and keep an eye out for where and when they try to go in.

If it's a groundhog, and the area it lives in is well-traveled, you should take care of the problem. We had groundhogs for years and years, attracted by the crabapple trees. After about 7-8 years or so, we had two broken ankles and a broken front axle on the riding lawn mower because the tunnels started collapsing.

- Lisa

We made a pen for our rabbit neat the storage shed and he was tunneling under the shed.

I have tried mothballs for all pests and they never work--don't waste your money. By the way where do you buy fox urine--I'd like to try that--I have a critter (must be a woodchuck)that ate a huge hole through my plastic and chicken wire fence to get to the beans--hope it had a major tummy ache.

Geri

Don't waste your money on fox urine ... I tried it ... doesn't work.

The fox urine worked for me. I had a woodchuck under my shed this spring who mowed all my chard,lettuce, and spinach seedlings and trampled a daylilly. I put the fox urine powder on paper plates and slid them under the shed and flung some cat poop and cat hair under there and he moved on. I also read that shiny, moving things bother them too so I put some kids windmills around the yard and haven't seen him since. p.s. All the plants recouperated except for the trampled daylilly (it bloomed sort of sadly) but I had a great harvest!

Put the Rat posion in the shed and lock it when your not using it. Also if you find a hole in your yard put your garden hose in it and go inside.(Might take awhile to fill den/flood other holes.) This will flood the den,(cruel but effective.) This will also show you where the other holes are so you and your neighbors can fill them.

(Best bet is to use small pebbles/rocks then pack dirt over it, or cement the top of the pebbles/rocks with
quick creat, if nothing else put a heavy rock over the filled hole so they cant get to it.)

Anything to keep them from re-digging their way back in.

Exactly how big are these holes?

Keep in mind that burrowing animals learned a long time ago how to make tunnels that would not flood, because if they did not learn that they would not be here today. Flooding any tunnel is not only a waste of your time and energy but a waste of water also.

I usually only had one or two chipmonks and one little hole, this year I got two new kittens and wanted them to watch the birds from the patio, so I fed the birds all winter, I now have a tunnel under the right side of my shed and they poke their heads through one tunnel and then race to the other, just like in a cartoon. I can't stand it. I quite feeding the birds 1 month ago, and they must be upset. I need to fill in the wholes and put moth balls or something and cement. I do not think they are cute, because in the past 40 years, we have spent money on cement around our porches, steps etc., retaining walls, to much damage, they are just rodents, so maybe soon I'll put the kittens out to fetch some of these critters. I am an animal lover, however, when the property keeps getting damaged, that's enough for me!!!!!!!!!!!!!!!!!!!!!!