Amager Barkke, when completed, will be cleanest waste-to-energy plant in the world. It will be the tallest and biggest building in Copenhagen. Now under construction, it will contribute to Copenhagen's ambitious goal of becoming carbon-neutral by 2025. When finished in 2017, it will produce heat for 160,000 households and electricity for 62,500 residences, fueled entirely by the city's garbage. It will house Denmark's first ski-slope (on the roof of the plant, no less). It will emit its CO2 emissions - not as a continuous stream of smoke, but in sudden, bursting smoke rings (each ring representing a ton of CO2 emitted). As an urban "destination in itself" and a landmark in environmental design, it's one of the most radical representations of architecture as a means of public engagement of our time.