EDIT: Didn't actually search for the topic before posting, sorry if was already discussed.
So I work in Center City, had to go to the post office today in the morning, and when coming back from there (suburban station) I took a hallway I usually don't take, and stop to look a SEPTA map that showed the underground concourse in full extension.
I wasn't aware it covered as many blocks.
For some reason is hard to find online (not on SEPTA website), but found a couple here:
Philadelphia Subway Concourse Map
I'm just wondering, has anybody walked from one end to the other, both north-south, and east-west, the whole concourse?