Nonprofit Flussbad Berlin has proposed a remarkable brownfield rejuvenation project for the center of Berlin: transforming an underutilized canal into an enormous outdoor natural swimming pool for summer recreation. And in winter, the pool would serve double duty as a skate rink. The canal, which flows along one side of the city’s UNESCO heritage-listed Museum Island, would be filtered by gravel beds to ensure public health and safety in the water, and the natural swimming pool, known as a Flussbad, would be a whopping 750 meters in length.