I had an underground varmint about ten years ago. I read where they are pretty territorial so you might only have one critter doing the damage. They can tunnel around 8ft per day when active.

I was lucky to catch it when it just started to dig a new burrow and stuck a smoke stick in the hole behind it. Minutes later it came out and croaked on the sidewalk.

Your critter might be eating earthworms. If you had grub worms there would be patches of dead grass in the yard. Grub worms eat the roots of grass and the dead grass would easily come loose if you pulled on it.

There are bottled liquids you attach to the garden hose to spray for burrowing animals that are pretty effective. Home Depot should have them. I knew I had grub worms so I used this product.