The design of the Metro was heavily influenced by Montreal's winter conditions. Unlike other cities' subways, nearly all station entrances in Montreal are set back from the sidewalk and completely enclosed; usually in small, separate buildings or within building facades. They are equipped with swivelling "butterfly" doors meant to mitigate the wind caused by train movements that can make doors difficult to open. The entire system runs underground and some stations are directly connected to buildings, making the Metro an integral part of Montreal's Underground City.