The final piece of the Greater Bay Area's 6,845-meter-long subsea tunnel was attached on Sunday in south China's Guangdong Province, completing the tunnel from end to end, according to the Guangdong Communications Group.

It is the world's longest and widest undersea steel shell concrete tube tunnel and is part of the 24-kilometer Shenzhen-Zhongshan cross-sea link project.

How was the tunnel connected?

The tunnel part of the link project consists of 32 tubes, and each standard tube is 165 meters long and weighs 80,000 tonnes. They were transported by floating them 50 kilometers along the Lingdingyang Channel on using a specially engineered vessel.

The vessel, "Yihangjinan-1," is currently the world's largest and most advanced ship for undersea tube transportation and installation, with the highest installation capacity, highest placement accuracy, and the most efficient performance. It reduces occupation time on the Lingdingyang Channel by more than 80 percent, temporary channel dredging by more than 70 percent, and improves the efficiency of tube transportation and installation by more than twice that of traditional technology, according to a China Media Group report.

The standard underwater tubes were dropped into place from the bottom of the vessel and aligned with others. But the last tube module, E23, required innovative prefabrication, including a mechanical joint.

After section E23 was placed in the right attitude, a hydraulic jack pushed the joint affixed to E23 outward, connecting it to the end of tube E24.

It's "like a gigantic drawer," said Zhong Huihong, deputy director of the Shenzhen-Zhongshan Link Management Center, adding that the maneuver allows them to utilize the equipment's advantage, reduce construction risks and ensure the reliability of the project.

Apart from the floating transportation and joint processing, another key aspect of the undersea tunnel's construction is the foundation bed treatment, which includes trenching, treatment and leveling.

The Greater Bay Area express link

The link connecting the cities of Shenzhen and Zhongshan will play an important role in developing the Guangdong-Hong Kong-Macao Greater Bay Area.

Together with a series of bridges, artificial islands, and a tunnel, the link forms an above-and-under-water transportation system that is deemed one of the world's most difficult cross-sea cluster projects.

The bridge sections of the link, which extend for 17.2 kilometers, have already been finished.

With both the tunnel and bridge sections of the link now joined, the project still has road development, electricity setup, safety checks and other installation works before it is ready to open for traffic by 2024.

The link allows people to commute between the two cities in around 30 minutes, down from 90 minutes before, forming a "half-hour transportation circle" for the two.