An aerial drone photo taken on March 31, 2026 shows a view of the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province. This 3.118-kilometer underwater tunnel, the first of its kind in Sanya, is expected to further optimize the urban spatial layout of the city, and facilitate the development of the Hainan Free Trade Port. (Xinhua/Yang Guanyu)

A staff member works at the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, March 31, 2026. This 3.118-kilometer underwater tunnel, the first of its kind in Sanya, is expected to further optimize the urban spatial layout of the city, and facilitate the development of the Hainan Free Trade Port. (Xinhua/Yang Guanyu)

This photo taken on March 31, 2026 shows a view of the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province. This 3.118-kilometer underwater tunnel, the first of its kind in Sanya, is expected to further optimize the urban spatial layout of the city, and facilitate the development of the Hainan Free Trade Port. (Xinhua/Yang Guanyu)

An aerial drone photo taken on March 31, 2026 shows a view of the construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province. This 3.118-kilometer underwater tunnel, the first of its kind in Sanya, is expected to further optimize the urban spatial layout of the city, and facilitate the development of the Hainan Free Trade Port. (Xinhua/Yang Guanyu)