A worker conducts checks on a tunnel boring machine at a construction site for the Jintang Undersea Tunnel in Zhejiang province on Thursday. YAO FENG/FOR CHINA DAILY

Tunneling operations began at both ends of China's longest undersea high-speed railway tunnel project on Thursday, according to China Railway's 14th Bureau Group, which is overseeing its construction.

The Jintang Undersea Tunnel, a critical component of the high-speed railway connecting Ningbo and Zhoushan in Zhejiang province, will span 16.18 kilometers upon completion, with a shielded section 11.21 km long, making it one of the world's longest undersea railway tunnels. Trains will travel through it at speeds of up to 250 km per hour.

Construction of the tunnel is one of the world's most ambitious and challenging projects of its kind because of the geological conditions it faces, the immense pressures at depth, and its length, which is nearly 10 km longer than the tunnel on the Hong Kong-Zhuhai-Macao Bridge.

Tunnel boring machines digging simultaneously from Zhoushan and Ningbo are set to meet in the middle of the seabed, delving as deep as 78 meters beneath the water's surface during construction.

It will traverse the Jintang Channel, a major water transportation route to Zhoushan Port in Ningbo. The channel can accommodate large vessels, and Zhoushan Port is one of the world's busiest ports in terms of annual freight throughput.

The tunnel will navigate a complex marine environment, passing beneath various risk factors such as oil pipelines, sea walls, docks and shipping lanes.

Hu Hao, an engineer from China Railway's 14th Bureau Group, underscored the exceptional challenges presented by the project, which include navigating 28 transitions between soft and hard strata.

"The construction complexity, difficulty and risks involved in this process are rarely seen in the world," he said.

Hu said the team has enhanced the design of the tunnel boring machines specifically for the Jintang Tunnel, enabling them to handle both soft and hard strata.

Upon completion, the tunnel will be the world's third-longest undersea railway tunnel, trailing the Channel Tunnel linking the United Kingdom and France and Japan's Seikan Tunnel. However, trains in those tunnels travel at a maximum speed of just 160 km/h.

Zhoushan and Ningbo are currently connected by a cross-sea bridge and ferries, which are subject to disruptions from typhoons, maritime transportation challenges and holiday congestion, resulting in erratic transportation capacities and inconvenience for commuters.

The Jintang Tunnel will be a crucial segment of the 77-km Ningbo-Zhoushan High-Speed Railway, which is expected to be operational by 2028.