We humans continue to sprawl across the planet's surface, bulldozing the forests and mountains in our way. But it doesn't have to be that way, argues Timelinks, a sustainable design firm in the United Arab Emirates. The firm's Ziggurat is "a building-city where people, nature, and modern technology are united" in a single structure measuring about one square mile at the base, which the company says could house up to 1 million people.

Residents would commute from their living quarters to their jobs within the Ziggurat using a public transit system that's part elevator, part tram. Clean energy will be produced in sundry ways, including power generators that harness the kinetic energy of sewage flowing down through pipes. And when residents get tired of the Ziggurat, they can venture beyond its walls to the unspoiled ecosystems surrounding it.