Easiest rat trap in the world. Take 5 gallon bucket. Lean it on a fairly steep angle, say 45 degrees. Make it possible for a rat to enter at the top. It could be propped on a hillside, for instance. Put enough water in the bottom to drown a rat. Go away. Come back, find drowned rat(rats) in the bottom. Dispose of rat. Do this again. Rats can't climb back up the slick side.
