Never mind using boilers to heat buildings in Paris. The city has found a novel way to keep a public housing project warm - body heat. The housing complex, now undergoing renovation, connects to the Metro subway system - and therein lies the source of the heat: Metro passengers.

In actuality, the system is capturing more than just the human calories. Body heat is combined with the heat generated from trains moving along the tracks. Combined, the subterranean location produces year-round below-ground corridor temperatures between 57 to 68°F (13.8 to 20°C).

Using geothermal technology, the project draws the heat, moves it first to heat exchangers and then to heating pipes. By using this method to make the rehabbed building environmentally friendly, carbon dioxide emissions will be cut by a third compared to what a boiler would generate, according to city officials.