A drone photo shows the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, Jan. 11, 2025. As the first underwater tunnel in Sanya, the project undertaken by China Railway 20th Bureau Group is making steady progress. It will further optimize the spatial layout of Sanya City once completed. (Xinhua/Yang Guanyu)

A drone photo shows the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, Jan. 11, 2025. As the first underwater tunnel in Sanya, the project undertaken by China Railway 20th Bureau Group is making steady progress. It will further optimize the spatial layout of Sanya City once completed. (Xinhua/Yang Guanyu)

A worker works at the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, Jan. 11, 2025. As the first underwater tunnel in Sanya, the project undertaken by China Railway 20th Bureau Group is making steady progress. It will further optimize the spatial layout of Sanya City once completed. (Xinhua/Yang Guanyu)

Workers work at the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, Jan. 11, 2025. As the first underwater tunnel in Sanya, the project undertaken by China Railway 20th Bureau Group is making steady progress. It will further optimize the spatial layout of Sanya City once completed. (Xinhua/Yang Guanyu)

A worker works at the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, Jan. 11, 2025. As the first underwater tunnel in Sanya, the project undertaken by China Railway 20th Bureau Group is making steady progress. It will further optimize the spatial layout of Sanya City once completed. (Xinhua/Yang Guanyu)

An aerial drone photo shows the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, Jan. 11, 2025. As the first underwater tunnel in Sanya, the project undertaken by China Railway 20th Bureau Group is making steady progress. It will further optimize the spatial layout of Sanya City once completed. (Xinhua/Yang Guanyu)