One of the most visible facilities in the city of Chicago is also one of the most mysterious. Every day, thousands of tourists flock to Navy Pier, and from its Ferris wheel, its restaurants, and its miniature golf course, wonder what the deal is with that big, heavily-guarded park to the north.

This is where Chicago's drinking water enters the city. It originates several miles offshore, and is brought to the filtration plant by a pipeline under the bed of Lake Michigan. The plant processes nearly a billion gallons of water each day for the thirsty metropolis, which includes Chicago and more than a hundred suburban communities. It takes seven hours for water from the time water gets sucked into the intake cribs until it squirts out of someone's hose or faucet somewhere in Chicagoland.