Question
I have holes in my backyard possibly by moles ??  What is the best method to fill the holes?  Should I simply put soil in the low spots then grass on top.  The reason I ask this question wouldn't the soil just sink?  
What are your thoughts?

Answer
Moles get blamed for all the ills of the world.  Don't rule out damage by Gophers, Deer, Squirrels and Racoons.

It's easy to mix up Gophers and Moles.  Gophers are bigger; their tunnels are larger and deeper.  They're also Vegetarians.

Moles are Carnivores (they are crazy about insects and Earthworms).  They show up a lot in regions where it rains a lot.  You find Gophers in drier areas.

If you have a healthy garden, you will have plenty of Earthworms, and the normal insect population that goes with the balance of Nature.

The myth about Moles is that they are attracted to Grubs, and that if you have Moles, you must have a Grub problem.  But Earthworms and other insects make up most of their REAL diet.

Rat traps are sometimes recommended to catch moles.  But there box and Maccabee Gopher traps are very effective.  Farm supply stores usually carry them.  If you have to dig below the ground down to the main tunnel, keep in mind that this can be 1-2 feet down.  The newest tunnel is usually the one that will catch your mole.  You can use carrot tops and other fresh greens as bait.  Check the traps daily or more. You have to be persistent about this.

Flooding the tunnel with a garden hose, or filling it with smoke from smoke bombs or a road flare, can pursuade Gophers and Moles to move out of your lawn.  Flooding is popular because it is so easy, but it is not considered very permanent most of the time, unles the poor Mole happens to drown.  Smoking them out is much more effective.

You will find special rodent smokebombs on the market that work fairly well. Even better are long-burning road flares, which you can pick up at any auto supplies store.  Light the flare carefully and stuff it underground, into the tunnel, then shovel dirt back over opening and step on that to seal the tunnel.

Watch the rest of the lawn for the escape route at the other end of the tunnel; when you spot it, plug up that end of the tunnel.  Do this over several days, wherever you see a tunnel.  Diligence usually pays off.

Getting these varmints to set up house in someone else's real estate is sometimes as easy as buying a cat.  Some people swear by garter snakes, although these can be hard to come by.  Some people sprinkle USED Cat litter, Dog poop, or urine (from whatever source you can find) at the tunnels.  This is a psychological deterrent with DANGER written all over it.  Rodents are sent running.

Human hair and flavored bubble gum are also considered Mole-hostile.  How effective they are depends on the Mole(s).

In my opinion, a diligent, multi-weapon assault would be effective.  It's a war of wills when you have these.  Shock And Awe, my friend, is the answer.

You don't need to fill the holes, by the way.  Simply rolling over them will quickly level the Soil.  With time, the Soil settles.  It does not sink, but don't ask me why.  It just does not seem to collapse the way you'd expect it to -- unless it's extremely Sandy, in which case I am GUESSING that it would just fill right in.  But don't quote me.  Good luck and thanks for your question, Brad -- keep in touch!