Runners take part in a race along the walkways on the roof of the National Stadium, also known as the Bird's Nest, in Beijing on Saturday, February 11, 2017. The newly renovated walkways have been extended from 200 to 1,000 meters in length, to cover the whole roof from south to north. The Bird's Nest has become the first Olympics main stadium in the world to opens its entire roof surface to the public. [Photo: people.cn]