The two-decade war between my Dad and the local foxes cannot be understated. The chickens are fully enclosed, naturally. They currently have a (completely buried) overturned concrete igloo under their feet. There’s a dual perimeter fence, the outer one is regularly coated in all manner of larger mammal’s urine he buys online. Team Fox is currently tunnelling to map out the concrete igloo, convinced there’s an opening. They’ve gone full mole.

With some distance it’s quite amusing, but it’s claimed a large part of his life, being the obstinate bugger he is.

Pea gravel. Lots of pea gravel in the holes. Blow it in with water.

You can't tunnel in pea gravel.

That's awesome. My father has been waging a similar war with the coyotes in the woods behind our farm for maybe 5 years now. Your foxes sound way more intrepid though, the coyotes here haven't tried burrowing, yet...