In better times City 17 had many, many ways of crossing town. For the most part, the citizens of today travel on the surface, and mostly on foot. This is partly because they're easier to track down, and partly because the alternatives haven't been maintained lately. On the other hand, the Combine don't use the underpassages any more either. Clearing one of the tunnels that crosses half the City is a priority for the Resistance; it'll give them a much better crack at getting behind enemy lines. Thus the infiltration team finds themselves in a small courtyard at the back of what was once a highway maintenance building, facing a poorly lit flight of stairs leading down.