It can be used on the bottom, that was the method for the person who first bred horned frog. I don't even think they burrow to stay cool or warm but somewhere in between when whether isn't ideal above. But whatever way used it should provided a thermal gradient and should be controlled by a thermostat. Personally I don't like heat mats when high temperatures are needed, they're more of a background heat source or night time IMO as they don't do much to air temperature. Heat mat on the side is great if only little heat is required though.