The last pour of concrete for an immersed tube underwater tunnel, a key part of China's new cross-sea highway project linking the cities of Shenzhen and Zhongshan, was completed on Tuesday.

The Shenzhen-Zhongshan Link is a 24-kilometer-long highway with a designed speed for vehicles to travel 100 kilometers per hour. It's a core transportation hub project in the Guangdong-Hong Kong-Macao Greater Bay Area.

It consists of an underwater tunnel, two bridges and two artificial islands, making it one of the world's most challenging cross-sea cluster projects.