Whatever they are. Some rodent had dug tunnels beneath and thru my patio area and apparently, beneath the edges of the pond. I had to pull up the liner and redo the upper part of the pond. I’m going to try using some cedar mulch beneath the liner, to see if that might deter the rodents (and the bugs they eat). That being said, the pond is almost full of water, and ready to accept the fishies. I’ll put them out in the next day or so. Setting up a mini pond above, to trickle in & bio filter, so it depends on how long that takes me.