On very hot days animals will often find a patch of dirt on the ground too lay and keep cool which best explains how laying on the ground in dirt keeps the animal cool

Laying in the dirt or in most cases mud, will cool an animal by bringing its temperature down. Usually dirt is already cool, if a rain hadn't already wetted it. If a pig lays in the mud, they love to sprawl on their belly which actually helps them to cool down. Blood vessels closer to the skin on a pig's belly carry the cooled blood throughout the body and the pigs temp goes down. It's kind of like with a jackrabbits ears and how the blood vessels are close to the skin.