I had occasionally made use of the subway concourse to duck out of the rain or cold for a block or two, but I had no idea how extensive they were. I found an old wall map in one passage and used it to make a simplified map: Philadelphia Subway Concourse Map
I can get to Suburban Station, Lord & Taylor, or Market East Station entirely underground. And because Market East Station has a shopping mall with pedestrian walkways over the streets, I can walk to Kmart, 9 blocks away, entirely inside.
I recently saw a guy lead three women down underneath the streets to get out of the cold. The women complained that it smelled bad, which it does a little, but I'll take that over getting soaking wet or freezing. But it's more than just the convenience of this underground labyrinth that intrigues me. I'm kind of surprised that I don't see more people down there. I know that other cities have street musicians and vendors underground, or even homeless people, trying to keep warm. I remember Munich's subway concourse was particularly lively, with a walkable shopping mall between stations, entirely underneath the old city above. I guess I have seen a little of that here, but just not nearly as much as I would expect, given the expansiveness of these passageways. The concourse under Broad Street is, well, broad: you could play soccer down there (just look out for the support beams). There's a lot of empty space underneath the streets.