Get yourself a Rodenator (
www.rodenator.com) to get rid of underground pests. It's ridiculously entertaining. At least look at the videos they have on the site.
Seriously, find the holes, and pick up some anti-rodent smoke bombs from a local hardware store. Light it, stick it in the hole, and pack with dirt. Anywhere you see smoke rising, fill in, and repeat the process a few times several hours apart to make sure they're all dead.