Walking around Philadelphia, you may come upon an occasional manhole cover stamped with STEAM, a Round marker of Center City's district steam system. Built by the Edison Electric Light Company in 1889, the system is a vast network of pipes that run under sidewalks throughout the city, delivering pressurized steam at a constant 450 degree temperature from various generation plants to hundreds of buildings for heating, cooling and hot water. Still in operation today, the 33 miles of pipe make this the third largest district steam heating system in the United States.

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