Hibernation sites are places where animals go to rest and conserve energy during the winter months. A CAVE, with four letters, is a large, natural opening in the ground or in a cliff face. A LAIR, also with four letters, is a DEN or resting place of a wild animal. Lastly, a DEN, with three letters, is a small, secluded shelter or hiding place. All of these locations provide a safe and secure environment for animals to hibernate during the winter months.