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A ceremony was held on July 7 for the Hong Kong-Zhuhai-Macao Bridge undersea tunnel to mark the completion of the main structure of the world's longest cross-sea bridge. The Hong Kong-Zhuhai-Macau Bridge is a super large sea-crossing passage, 55 km in length. The most difficult technique is the main bridge-island-tunnel project, which is about 29.6 kilometers long. [Photo/Xinhua]