A behemoth development of hotels, residences, restaurants, stores, and casinos may seem an unlikely paragon of sustainability, but the year-old CityCenter is anything but conventional. Its LEED-certified hotels (the Mandarin Oriental, Vdara, and Aria Resort) were constructed with cutting-edge green-building techniques. Classic Vegas amenities have been given a new twist: limos run on natural gas; slot machines double as air conditioners. But it's the sheer scale of CityCenter's accomplishments that's most notable. The development's efficiency initiatives alone save an estimated 94 million kWh of energy and 50 million gallons of water each year.