Workers prepare pipe curtain machine "Lucheng" for jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 29, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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)

Workers prepare pipe curtain machine "Lucheng" for jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 29, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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)

Pipe curtain machine "Lucheng" is about to start jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 29, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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/Zhao Yingquan)

Pipe curtain machine "Lucheng" performs jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 30, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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. (Photo by Qian Lei/Xinhua)

Workers prepare pipe curtain machine "Lucheng" for jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 29, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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)

Pipe curtain machine "Lucheng" is about to start jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 29, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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)

Pipe curtain machine "Lucheng" performs jacking operation at a construction site of the Sanya River estuary passage project in Sanya, south China's Hainan Province, April 30, 2026. The pipe curtain machine "Lucheng," capable of combined jacking of interlocking triple pipes, started jacking operation here on Thursday.

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. (Photo by Qian Lei/Xinhua)