The frost line is the level down to which the soil will normally freeze each winter in a given area. Building codes must take this into account, as foundations must be dug down to or below this point. Failure to do so will cause frost heave to dislodge the building at least slightly, causing damage which may become a serious threat to the building's structural integrity.
In Chicago and other locations of similar climate, the frost line is six feet, or 1.8 meters. In near-Arctic and Antarctic locations, the frost line is so deep that it becomes year-round permafrost, and there is instead a "thaw line" during the summer.