also, the two abandoned segments of tunnel under arch street between 10th/11th streets and 12th/13th streets would be crazy to see. they were abandoned when World War I began. i've only been able to find 2 pictures online, however on Google Street view you can see possible ventilation grates between 12th and 13th streets
looks like almost nobody has been down there, as the usual graffiti and homeless trash are missing. however, the new cinderblock support pillars, wooden steps, and empty soft drink cup in left hand corner show somebody has been down there
edit: just found a 3rd pic taken in 1952