The 3rd International Holcim Awards for sustainable construction projects from across Europe were recently announced at a ceremony in Milan, with the German architectural firm realities:united taking first prize for its Flussbad proposal. Led by architect Tim Edler, Flussbad would transform a stretch of the River Spree in Berlin into a 745 meter (almost 0.5 mile) -long "swimming pool." That's about the size of 17 Olympic pools! It is hoped that the project will provide Berlin residents with a new way to use the city's historic center, whilst taking advantage of a currently unused section of Berlin's inner city river.

The renewal concept for the 3.9-hectare (9.6-acre) site is "surprisingly simple and very cost-efficient" says realities:united. Parts of the Lustgarten quay would be transformed into stairs, creating an access point to the swimming pool, recreation gardens and functional facilities such as lockers, change rooms and bathrooms. The plans incorporate an upper 1.8-hectare (4.4-acre) reed bed, which would naturally purify the water entering the swimming pool, whilst a barrage at the lower end of the system would prevent backflow of unfiltered water from the river.

The conversion will not only create the world's longest swimming pool, but would also create a popular attraction amidst the historic center of Berlin. The Holcim Awards jury commended the proposal as "an excellent example of what could be achieved within challenging inner city areas that possess a rich tradition and cultural heritage where the local public has been overlooked."

A group of entrepreneurs have proposed a similar project, the +Pool, for the rivers in New York City.