These dens are also very long, up to 45 ft of tunnels, extending as far underground as 5 ft. Dens in open areas are used in the summer, and those under stumps, at the edge of rock ledges, and near other protection are generally occupied during the winter hibernation. Dens are always kept clean and well padded. Occupied dens can be recognized by a pile of fresh earth at the entrance. Nesting chambers are about fifteen inches in diameter and are usually sparsely lined with dry leaves.
Woodchucks are true hibernators, relying soley on body fat for winter survival.
Hibernation takes place in a side chamber along one of the underground corridors of the burrow system. While hibernating, the woodchuck rolls itself into a ball and tucks its head between its hind legs. Hibernating chambers are thought to be located under the roots of trees. Woodchucks somehow seem to know when to wake up, controlled by a circannual clock, but actual emergence depends on the daily temperature. Soon after leaving hibernation, sexually mature woodchucks begin the reproductive process.