Believe it or not, the world's largest green roof sits on top of two underground parking garages and an active commuter rail yard in Chicago.
Millennium Park, in downtown Chicago, was filled to the brim when hundreds of thousands of people celebrated President Obama's election in 2008, and the award-winning outdoor center hosts many of the cities most prestigous cultural events.
You'd never know it's a roof - it looks just like a park, with winding pathways, seating for 11000 people who attend symphonies, and 2.5 acres of native plants.
The 24.5 acre, 99,127 square meter green roof sits on top of Millennium Garages. It extends over the parking garage at Soldier Field, home of the Chicago Bears football team - the recipient of a Green Roofs for Healthy Cities Award of Excellence in 2004.
At a cost of $500 million, it's the brainchild of former Mayor Richard M. Daley, and one of the first public/private partnership construction projects in the city.
Millennial Park's green roof is 20,000 square meters larger than the second largest green roof in the world at Frankfurt International Airport in Frankfurt, Germany.
Chicago now boasts more green roofs than any US city and some of its largest are built over underground structures. It has more than 200 green roofs, covering a total 2.5 million square feet. A green roof tops Chicago's City Hall and Cultural Center, which also have beehives.
A green roof is an ecological workhorse, reducing storm water runoff and energy consumption. It provides acoustic protection, creates habitat for birds and insects, and is key to mitigating the urban heat island effect.
Although the cost is higher upfront, the payback over time for green roofs is significant in energy savings and in protecting the roof membrane, making it possible to skip up to three replacement cycles.
In Bronx, New York City, a 36,512 square meter golf driving range is even considered a green roof - it's being built on top of the Croton Water Filtration Plant, which is sited on a golf course. It's expected to open in 2014.
And the largest green roof in Australia is also being built on top of a water plant, reports Engineering News-Record. The Victorian Desalination Plant near Melbourne, Australia will host a 26,000-square-meter green roof next year. It is being planted with natives that will camouflage the water plant, while providing corrosion resistance, thermal control and acoustic protection.
Stuttgart, Germany, requires green roofs on all new flat-roofed industrial buildings. In the US, Portland, Oregon, requires new city-owned buildings and existing buildings that replace roofs to install a green roof on at least 70% of the area.
Learn more about Chicago's green roofs.
Here's a slide on the world's biggest green roofs: