Copenhagen is recognized as one of the most environmentally friendly cities in the world. As a result of its commitment to high environmental standards, Copenhagen has been praised for its green economy, ranked as the top green city for the second time in the 2014 ''Global Green Economy Index (GGEI)''. In 2001 a large offshore wind farm was built just off the coast of Copenhagen at Middelgrunden. It produces about 4% of the city's energy. Years of substantial investment in sewage treatment have improved water quality in the harbour to an extent that the inner harbour can be used for swimming with facilities at a number of locations.