Copenhagen, Denmark After successfully evolving into a cycling city, in the 1990s Copenhagen set its sights on becoming a year-round destination for urban swimming. The only thing standing in its way was the toxic harbour, which for years had been blighted by industrial waste and sewage. But pollution once did: for a long time, the city’s harbour was awash with toxic swill from the city’s now-defunct shipbuilding industry. For a long time, the city’s harbour was awash with toxic swill from Oslo’s now-defunct shipbuilding industry. It’s not the only Swiss city to bring urban swimming back: Bern, Geneva and Zurich have also done so.