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It's a shocker -- one of the most expensive homes in Chicago was once an electrical substation. Now an eco-friendly home for a buyer looking to settle in the Windy City, the converted classic is listed for $12.5 million.

It's easy to spot the former Commonwealth Edison building from the outside, as the structure retained its brick Art Deco faade from a century ago. But the modern renovation is also apparent, lurking a bit behind the original two-story structure. Two new modern levels visible from the street hint at what this fascinating piece of property holds.

"You'd never assume there's a tremendous house there -- it's very laid-back, very Chicago-style, very low-key," listing agent Chezi Rafaeli says of the six-bedroom home wedged between two townhomes on Clark Street.

Chicago attorney Bruce Gelman and his wife purchased the substation in 2008 for $3.75 million; exactly how much money they poured into the transformation is undisclosed, but the job took almost four years to complete, according to Rafaeli.

To convert the substation into a sublime living space, the couple enlisted local architect Michael Hershenson. His vision expanded the building to four stories and 15,000 square feet, with a central atrium skylight.

It features two kitchens, office with a limestone fireplace, 10-foot-long shower ("It's like another bedroom," Rafaeli notes), wine cellar, exercise room, playroom, solar heating, four-car garage, and rooftop greenhouse overlooking Washington Square Park. What used to be a parking lot is now a courtyard with a 25-foot-long pool.

The home has bounced on and off the market since spring of last year, with fluctuating prices and with a different listing agent. It was listed for $15 million in July 2015, relisted for $13.5 million last month, and now sits at $12.5 million. Rafaeli says the owners are motivated to sell and doesn't think they'll have much of a problem finding a buyer.

"When you enter into the house through a corridor, you see this giant outdoor pool -- which is very cool to see when you're right in the city, seeing a place with this much outdoor space," he says. "It's a mind-blowing house, no question. The amount of glass, the soaring ceilings, and it's eco-friendly."