Snakes are reptiles which means that they are literally cold-blooded and need to find a way of surviving during the winter. What they do in order to meet this objective is just getting under the ground level by staying inside a den.
How does this helps them ?
The cave effect.
The cave effect corresponds to the physical phenomenum also used by the french to conserve wine in wine caves and by plants to allow their seeds to germinate. The physical characteristics of the ground decrease the temperature variations and also change its phase as we go beneath it. It's even possible to manage to reach phase opposition with the outside temperature in a certain depth, which allows you to have summer temperature conditions inside a cave or den while it's winter outside. This is why our building is partially built underground and why we have a grass roof. This way we can control the temperature inside the building and consume less electricity for warming and cooling it, which corresponds to the most energy consuming activities in every european building.