The wolf den...a few facts :

Wolves use different places to rest; places with cover are preferred during cold, damp and windy weather, while wolves in dry, calm and warm weather readily rest in the open. During the autumn-spring period, when wolves are more active, they willingly lie out in the open, whatever their location.
Dens are often reused by generations of wolves. Some dens have been dated at more than 700 years old. On rare occasions more than one wolf will use the same den at the same time.
The odour of urine and rotting food emanating from the denning area often attracts scavenging birds such as magpies and ravens.
Though they mostly avoid areas within human sight, wolves have been known to den near houses , roads and railways.
Actual dens are usually constructed for pups during the summer period. When building dens, females make use of natural shelters such as fissures in rocks, cliffs overhanging riverbanks and holes thickly covered by vegetation.

Sometimes wolves will use abandoned dens of other animals, such as bear dens or a beaver dam.In a normal den, the birthing chamber lies at the end of a tunnel that may be up to 15 feet long in soft soil. The den is often slightly elevated above the rest of the tunnel.

On rare occasions, female wolves will dig burrows themselves, which are usually small and short. A den usually have 1-3 openings which are usually marked by a large pile of dirt. Wolves do not line their denning places, a likely precaution against parasites. The den is usually constructed not more than 500 metres away from a water source.
A wolf pup emerging from its den
Wolf pups playing close to the den
Interesting facts about wolves :

1.Wolves run on their toes. This enables them to hunt their prey at high speeds. Wolves normally run down their prey wounding it in the process. When the prey animal is exausted and weak they bring it down.
2.Wolves have about 200 million scent cells. Humans have only about 5 million. Wolves can smell other animals more than one mile (1.6 kilometers) away.
3.A wolf pup’s eyes are blue at birth. Their eyes turn yellow by the time they are eight months old.
4.Under certain conditions, wolves can hear as far as six miles away in the forest and ten miles on the open tundra.
5.Immense power is concentrated in a wolf’s jaw. It has a crushing pressure that measures higher than most land predators, ( More than 1000 pound per square inch compared with around 750 for a large dog). The jaws themselves are massive, bearing 42 teeth specialized for stabbing, shearing, and crunching bones. Their jaws also open farther than those of a dog.
6.Wolves have about a 20% success rate on average when hunting. Many die as a result of injuries sustained in the hunting process. In short, the prey animal wins 8 out of 10 times, dashing the myth that wolves hunt for fun.