HotSpring has a 60lb density foam bottom, It's hard as rock, but then nothing is going to enter from the bottom if your spa is on a firm foundation with 4-6000 lbs of water and spa pressing down on it, where a rodent can enter is into the equipment compartment where the access panel is. A HotSpring has a metal mesh rodent barrier installed to prevent this very issue, those Watkins guys think of everything.