The tunnel boring machine Dinghai starts its undersea excavation in Zhoushan, Zhejiang province. [Photo/Tide News]

The tunnel boring machine Dinghai crossed the sea wall of Jingtang Island in Zhoushan, Zhejiang province, to begin excavating from the land section into the sea section at a depth of 42 meters below sea level on Oct 22.

This marks the beginning of an "undersea journey" for the construction of the world's longest undersea high-speed railway tunnel.

As a key project of the Ningbo-Zhoushan Railway, the Jintang Undersea Tunnel is the world's longest undersea high-speed railway tunnel currently under construction, stretching 16.18 kilometers between Ningbo and Zhoushan. Of this, 11.21 kilometers will be built using tunnel boring machines.

The tunnel boring machines Dinghai and Yongzhou began their excavations from Zhoushan and Ningbo, respectively, on May 16. Dinghai plans to bore 6,270 meters westward, while Yongzhou plans to tunnel 4,940 meters eastward, navigating high water pressure and complex geological conditions.

Dinghai advances at an average of 16 meters per day below sea level, while Yongzhou has completed 902 meters of land excavation and is expected to enter the sea section by May next year.

"The tunnel will reach a maximum depth of 78 meters, facing water pressure of up to 8.4 pascal, equivalent to nearly a metric ton of weight on an adult's palm," said Zhang Jintao, project leader of the Jintang Undersea Tunnel.

The Ningbo-Zhoushan Railway will span 76.4 kilometers with a designed speed of 250 kilometers per hour. It will integrate Zhoushan into the national railway network.