Running from Maridalsvannet to the Oslofjord, this fast-flowing river was once the centre of Oslo's industry and then, until the late 20th century, abandoned and unloved. Today it's one of the city's favourite places to relax, an 8km swath of forested rapids, waterfalls, running tracks, picnic grounds, swimming holes and fishing spots. The crashing falls at Beier Bridge are an inner-urban wonder.