On some occasions, the river of time, anthropomorphically angered at being forgotten, floods the basement, or, alternatively, downtown Chicago:

The abandoned freight tunnels filled quickly, soon taking in about a quarter of a billion gallons. Water passed easily through old concrete barriers and soon began to fill the city’s subway system tunnels as well. Businesses that had forgotten about their illegal freight-tunnel hook-ups a half a century earlier were shocked to find their foundations filling with up to forty feet of water.The power grid began to short out, the Board ofTrade and Mercantile Exchange suspended trading when waters began to percolate up through their basements, and the entire downtown and financial district were eventually shut down and evacuated.

Lost urban undergrounds are an entire historical genre to themselves, whether they be unused bits of the London Tube, the (now touristy) ossuary under Paris, or a range of other oddities.

History isn’t always in the past, and sometimes it brings water-balloons.

P.S. And more links, courtesy of Jonathan: Cincinnati and Paris.