Vertical forest.

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The world's first forest-covered buildings are under construction in Milan, Italy. The twin high-rise apartment complexes are called Bosco Verticale, Italian for "vertical forest." The green structures will be covered with 900 trees and thousands of plants--the equivalent of 10,000 square meters (108,000 sq. feet) of woodland.

Irrigated with recycled water from the apartments, the foliage will improve air quality by filtering pollution and absorbing carbon dioxide, a gas produced when people burn fossil fuels thought to contribute to climate change. The plants will help residents conserve energy by providing shade in summer and letting light through bare branches in winter.

The forest in the sky will also create a habitat for birds and insects, which are disappearing from sprawling cities. "We have a building that is a world symbol for sustainable living," says lead architect Stefano Boeri.