A Polluted Canal In Berlin Could Transform Into The World's Largest Swimming Pool - We're talking 800-meters long

This will be a wonderful sports locale for triathletes and equally sublime center for open water swim races.

Recuses would be easier, spectating would provide spectacular views and the neighborhood is stunning:

From FastCoexist: 

The project would transform a side canal that has been closed to boat traffic for the last decade. At one end, a natural filter, made of layers of gravel and pollution-eating plants, would purify the river water flowing into the pool. A bypass would send sewer water in another direction, and the top of the structure would turn into a pedestrian and bicycle bridge. The beginning of the canal would be "re-naturalized" into a refuge for urban wildlife and a new park.

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