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CHICAGO -- Chicagoans often pride themselves on their in-depth knowledge of the city's secrets, from the shadowy nooks under Lower Wacker Drive to the old hangouts of Al Capone that aren't listed in guidebooks.

Yet even the most knowledgeable city dweller may be at a loss to explain those cylindrical stone structures sitting a few miles into Lake Michigan.

"People are curious," said Francis Blake, deputy commissioner of the city Department of Water. "They drive down Lake Shore Drive and have no idea what they're looking at."

On a clear day, the round, squat stone buildings are visible from Navy Pier. They're not tall enough to be lighthouses, and they definitely aren't oases for sun-happy boaters. They are water intake cribs, providing Chicago with most of its drinking water, and they have stood 2.5 miles out in Lake Michigan for more than a century, facing down storms and withstanding wind and waves.

Although it has been cleaned at the James W. Jardine Water Purification Plant, the water Chicagoans get when they turn on the faucet was probably sucked out of the lake by an intake crib, which stands in 32 to 40 feet of water.

Five cribs skirt the city's shoreline: the Wilson Street crib, the Harrison and Dever cribs off Navy Pier, the Four-mile crib off 22nd Street and the 68th Street crib. Of the five, only the Dever and 68th Street cribs are still active. Dever and Harrison are joined by a 100-foot bridge. The Wilson Street crib is a designated "standby" crib in case of an emergency, Blake said.

"Under extreme duress, like if one of the two active water tunnels collapsed, Wilson can be opened up in order to get water to the northern end of the city," Blake said. "It would be primarily for fire protection."

Instead of providing water, Harrison now provides housing for the maintenance crews that spend days at a time painting and performing repairs on the crib during the summers. The living quarters are simple, nothing more than what you'd expect to get with an efficiency suite at a beachside cottage. A set of stone steps leads up to a series of simple bunkrooms. In addition to bedrooms, there is also a comfortable kitchen, a pantry and a common area with a television.

Despite using toilets that incinerate bodily waste, swatting flies the size of small horses, contending with dead fish, bird droppings and high winds, the men who work on the cribs love their jobs.

"If you don't mind being isolated, it's a nice way to spend the summer," Blake said. "The men have a barbecue and they bring their fishing poles. They share the cooking."

Crib workers have experienced their fair share of strange experiences while stuck out in the lake at night. About five years ago, Blake related, two bricklayers were on the crib for a week-long repair job during the summer. It was a hot night and they had left the doors and windows open to get some air. At 2 a.m., one of them awoke to find a guy standing at the foot of his bed. Apparently the man's boat had capsized a few miles out and he swam to the crib, pulled himself up and went looking for help.

Once the men got over their shock, they gave the stranded boater blankets and coffee, and called a police cutter to come and get him.

Usually, the doors and windows are locked, Blake said, and the crib is "like a fortress." Casual boaters, however, occasionally make pests of themselves.

"Boaters do come out here on weekends, but they're not supposed to," said Ed Papelas, the marine pilot of the James J. Versluis, the boat that ferries men to and from the crib. "We'll come out here on Monday and find boats sitting around from the weekend. There are police patrols, though, that keep them from hanging around."

Until about 20 years ago, Blake said, there were men on the cribs 24 hours a day, 365 days of the year. Their job was to clear the screens over the water shaft of fish, debris and, in the winter, ice. After Jardine was built, the screens at the cribs weren't needed as much and it was no longer necessary to maintain a crew on the crib.

The Versluis, the only certified icebreaker on Lake Michigan, takes maintenance workers out the Dever crib once a day during the winter to monitor the ice buildup. Occasionally the weather will get so bad the men have to stay the night.

"There have been times where we've had to dynamite the ice," Blake said.

Water enters the crib through ports near the base of the crib, and rises around a central shaft. It flows through openings at the top of the shaft and down to the intake tunnels, which are located about 75 to 200 feet below the surface of the lake. Once the water flows through the tunnels to one of the two purification plants, it begins an eight-hour cleansing process.

After the water is cleaned, it's sent through three water mains to the 12 pumping stations around the city and 121 suburbs. The stations provide water to residences, buildings and fire hydrants through 4,200 miles of pipes.

The Jardine plant is the largest filtration plant in the world and it's likely to stay that way, said Adebola Fagbemi, a filtration engineer at the plant.

Jardine runs 24 hours a day and 365 days a year, using electricity to power the pumps that lift the water to be cleaned.

If the power goes out, Chicagoans would still get clean water, Fagbemi said. It just may not be aesthetically pleasing.

"We'd have to bypass the system and we'd be unable to add anything other than chlorine," he said. "The water would be safe to drink, but it might have an odd smell."

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