The 6,845-meter-long subsea tunnel of the Shenzhen-Zhongshan Link began its last tube installation on Thursday in south China's Guangdong Province, with the overall tunnel construction expected to finish in four days.

Carried by an engineering vessel for undersea tube transportation and installation, a customized ship developed for the mission, the final section of the tunnel and a junction part are expected to reach its destination by the early hours of Friday.

The tube, which is 165 meters long and weighs 80,000 tonnes, was submerged in water with most of its parts.

Featuring a two-way eight-lane design, the tunnel is the world's longest and widest undersea steel shell concrete tube tunnel. Its length of 5,035 meters consists of 32 tubes and a junction part, and 31 tubes have already been put in place from both ends.

"The subsea tunnel uses prefabricated tubes and is shaped like a gigantic drawer," said Zhong Huihong, deputy director of the Shenzhen-Zhongshan Link management bureau, adding that in this way, they can utilize their equipment advantage, reduce construction risks and ensure the reliability of the project.

World's most difficult cross-sea cluster project

The 24-kilometer Shenzhen-Zhongshan Link, a super sea-crossing project that comprises a bridge, an island, a tunnel, and an underwater channel, is expected to complete by November and open to traffic in 2024. Its 17.2-kilometer bridge has already been finished.

The link is an S-shaped route that considers multiple factors, such as hydrogeology, flood control and aviation height restrictions, resulting in a safer and more comfortable driving experience for commuters and tourists alike.

Once completed, the link will shorten travel time between the two southern China cities from two hours to 30 minutes.