Pocket Gopher most likely. I've gotten pretty good at trapping them. Find their most recent mound, get a probe, I use a 1/4" metal rod, and start poking the ground 12"-18" away from it feeling for a void about 8-10" down. If there is another mound a tunnel probably goes toward it. Once you find the tunnel, take a shovel, preferably a drain spade, and dig down. With the drain spade I can take a 10" diameter plug out in once piece. Stick your traps in the tunnel on each side of the hole. I use these:
https://www.traplineproducts.com/product-page/gophinator-gopher-trap-2-packDrop the plug back in the hole. You want to block off the tunnel so they go back to dig it out. Also make sure the plug fills the hole, no light going in. They're most active in the afternoon and at nights. I find if I set the trap in the morning, 80%+ of the time I have a dead gopher that evening. The mounds you see are where they shove the dirt out of their tunnels they dig to the surface. They plug these tunnels once they're done so if you probe too close to them you probably won't find a void.
