My friend with the whiskers would like to say “Hi!” He is a prime example of misaligning the pressure and thermal boundary. In any heated space, the boundary that holds the warm air inside of your home (pressure boundary) should be touching, and on the warm side of, your insulation (thermal boundary). In this case, the insulators put 6 inches of insulation in a 10 inch joist bay in a crawl space, and fastened it in such a way that it hung under the floor and made a cozy tunnel for various rodents.

This picture is inside the joist bay. The floor is above, the joists are to the left and right, and the insulation is at the bottom of the photo. Let me tell you, if I was a rat, I’d be exploiting this pressure/thermal misalignment too.
There are a number of ways to fix this problem. Poison, traps, and weasels are a temporary fix. Long term solutions include installing appropriately sized insulation, or removing the fiberglass insulation and creating a sealed, mechanically ventilated crawl with insulation on the foundation.