The network is particularly useful during Montreal's long winters, during which time well over half a million people are estimated to use it every day. The network is largely climate controlled and well-lit, and is arranged in a U-shape with two principal north–south axes connected by an east–west axis. Combined, there are of tunnels over of the most densely populated part of Montreal. In total, there are more than 120 exterior access points to the network, not including the sixty or so Metro station entrances located outside the official limits of the RÉSO, some of which have their own smaller tunnel networks. Some of the city's larger institutions, namely McGill University, the Montreal Museum of Fine Arts, Concordia University and the Université de Montréal, also have campus tunnel networks separate from the Underground City.