More than 100 meters beneath the seabed in southern China, engineers are advancing what is now the deepest undersea tunnel ever constructed for a high-speed railway. The project, carried out by the Chinese-built Shenjiang-1 tunnel boring machine, has already crossed the 4-kilometer mark after four years of continuous work.
The tunnel belongs to the future Shenzhen-Jiangmen high-speed rail line, a 116-kilometer connection expected to reduce travel time between the two cities to less than one hour. As reported by CGTN, excavation reached a record depth of 113 meters below the seabed in April 2026.
The work is taking place beneath the Pearl River Estuary, one of the busiest maritime regions in southern China. There, engineers face a combination of high water pressure, fractured geology, and multiple underground fault zones that complicate every stage of construction.
A Record-Breaking Tunnel Beneath the Seabed
The undersea crossing is located between Dongguan and Guangzhou and stretches for 13.69 kilometers beneath waterways linked to the Pearl River system. Information published by CGTN states that the tunnel reached 113 meters beneath the seabed earlier this year, establishing a world record for an undersea high-speed railway tunnel.
Engineers expect the maximum excavation depth to reach 116 meters. At these levels, hydraulic pressure becomes one of the project’s main technical constraints. The tunnel itself has a diameter exceeding 13 meters, making it one of the largest structures of its kind built under such conditions.
The broader railway corridor will extend across 116 kilometers once completed, directly linking Shenzhen and Jiangmen in southern China.
Shenjiang-1 Pushes Through Fractured Seabed Layers
The project relies on the Shenjiang-1 tunnel boring machine, designed and manufactured in China for deep undersea excavation. Over four years of uninterrupted work, the machine has progressed through highly unstable terrain beneath the estuary.
Chinese state media cited by CGTNreports that the route crosses 13 geological strata, five categories of composite geology, and six separate fault zones. These conditions expose the excavation system to constant pressure variations and mechanical stress.
To maintain stability underground, the machine uses two slurry circulation systems. One delivers fluid directly to the cutting head to reduce friction during excavation. The second transports excavated material back to the surface, where it is processed before being reused in the operation.
Underground Work Moves Forward
Behind the tunnel boring machine, crews install prefabricated concrete segments that form the tunnel lining. Each segment measures roughly two meters wide, and nine pieces are assembled to complete a full ring inside the structure.
This simultaneous process, excavation at the front and structural assembly behind, allows construction to continue without interruption despite the demanding underwater conditions. Workers carry out these operations continuously beneath the estuary floor.
“We are now at a stage where transport integration is prioritised, so various means of transport need to take one another into account,” stated David Feng, an independent Chinese railway specialist.
The tunnel is part of a larger infrastructure program connected to the Guangdong-Hong Kong-Macao Greater Bay Area. Once operational, the Shenzhen-Jiangmen railway will join China’s coastal high-speed rail corridor. China currently operates more than 45,000 kilometers of high-speed rail lines, the largest such network in the world.
Enjoyed this article? Subscribe to our free newsletter for engaging stories, exclusive content, and the latest news.