I discovered a rat problem under my pond and so I removed my preformed liner along with pumps etc. Found tunnels from one side to the other plus 2 homes at the far corners.the pond sits in a spot surrounded by 3 concrete walls 19 inches deep but the water fall side is just a dirt slope this is the entrance. I will dig this out and place crushed stone to the top and hope this stops them from entering..I also think I will fill the side up with crushed stone and after it is level with the top I will relay the slate on the top and hope that this works.i live in Ontario Canada, it will soon be cold but I am not wanting to house rats over the winter months. Any other ideas would be helpful too.thanks