Photo taken on May 13, 2018 shows sea of clouds after rainfall at Huangyaguan section of the Great Wall in Tianjin, north China. China's National Cultural Heritage Administration has vowed to strengthen the protection of the Great Wall, which is more than 21,000 km long. The Great Wall, a UNESCO World Heritage Site, consists of many interconnected walls, some dating back 2,000 years. (Xinhua/Yang Yushan)