Many, many tunnels
Chicago has both Boston and Brooklyn beat with SIX different sets of “left behind” holes in the ground: the Pedway, CTA tunnels, cable car tunnels, freight tunnels, water tunnels, and “The Deep Tunnel.” (Please stop smirking.) The cable car tunnels pre-date the “L," are 60 feet below ground, and went unused after the city switched to elevated trains to get over drawbridges (circa 1906). “The Deep Tunnel,” or Tunnel and Reservoir Plan (TARP), heads down 350 feet and is essentially there to accommodate excess rainwater runoff. The freight tunnels, unique to Chicago, were designed for tiny trains and were used to transport coal and freight between prominent buildings within the Loop (i.e. City Hall, Merchandise Mart).