This concept for suburban Keizer, Ore., called Nature City, includes a massive “compost hill” that harvests methane from yard waste, creates compost, and warms the swimming pools on top. | Credit: WORKac

Live in a cave! Or a tower with a waterfall inside! Cave dwellers live next door to the indoor climbing wall, while the waterfall generates water pressure for the whole development. | Credit: WORKac

This design, dubbed The Garden in the Machine, plops eco-groovy “vertical neighborhoods” and wildlife habitat into the space of an old factory in Cicero, Ill., a suburb of Chicago. | Credit: Studio Gang Architects

“Recombinant houses” within the neighborhood structure allow families to shift their living spaces to meet their changing needs. Kids go off to college? The neighbors can have the extra bedrooms. | Credit: Studio Gang Architects and Joseph Lekas Photography

The project, Thoughts on a Walking City, takes a suburban neighborhood in Orange, N.J., and turns it into a pedestrian-friendly "transit village" by putting multistory apartments where streets now lie. | Credit: James Ewing

How do you get around without streets? Check out those cute little bike and walking paths wedged between the houses and apartments. | Credit: MOS

The Simultaneous City project team designed a mixed-use downtown for Temple Terrace, Fla., including a new Town Hall, business incubators, and three types of housing. | Credit: Michael Bell, Eunjeong Seong: Visible Weather

A network of elevated houses is cooled by the prevailing winds, and provides shade to lower floors and public spaces below. | Credit: Michael Bell, Eunjeong Seong: Visible Weather