But out of sight is out of mind, and when the system ceased operating in 1969, the abandoned tunnels were forgotten by all but cable engineers.
And then, one Monday morning in 1992, the lights began going out all over the city.
Six months previously, cable-layers had come across an alarming sight where one tunnel passed under the Chicago River. A pile had pierced the tunnel roof and river mud was slowly forming a mound at its base. They videotaped the scene and sent the tape to City Hall. But having made its way to the relevant official, it lay in his desk drawer, forgotten like the tunnels themselves.
Another official, whose job it was to inspect the piling works, later admitted that he had not done so because he "couldn't find parking".
And so the water continued to drip into the tunnel - until April 13, when the city's luck ran out and the trickle became a torrent. By 7am, basements and sub-basements were filled with water 40 feet deep. And since the basement is where electrical, heating and air-conditioning equipment is located - not to mention valuable paintings in the case of the Institute of Art - the consequences were devastating.
Soon, downtown Chicago found itself without power. Office workers were trapped in lifts, the subway system was closed and radio and television stations were off the air.
Meanwhile, engineers were trying frantically to plug the hole in the river bed, at one point throwing hundreds of mattresses into the whirlpool.
Eventually, a team of mining divers sealed off the tunnel on either side of the breach, and the long job of pumping out the floodwater began. Even longer, though, was the job of counting the cost of the disaster. At $2 billion, it may well have exceeded that of Chicago's great fire of 1871.