An underground tunnel section of the long-awaited Hong Kong-Zhuhai-Macao Bridge was completed on Friday, marking the end of construction of the main structure for the world's longest cross-sea bridge, the bridge management authority said.

The 6.7 kilometer tunnel is 48 meters under the waters of Lingdingyang in the Pearl River Estuary - the longest and deepest underwater tunnel in the world.

The tunnel is comprised of 33 pipe-shaped sections linked by one 6,500-metric-ton butt joint in the middle. Each pipe weighs about 80,000 tons, as much as an aircraft carrier.

The 29.6 km main structure of the gigantic project also includes a 22.9 km overwater bridge, construction of which was finished last year.

Zhu Yongling, director of the management bureau of the bridge, said at a ceremony on Friday that the bridge will open to traffic by the end of this year following seven years of construction.

The Y-shaped bridge will cut travel time between Hong Kong and Zhuhai in Guangdong province from three hours to an estimated 30 minutes. Experts believe the bridge will play a significant role in the economic development of the Guangdong-Hong Kong-Macao Greater Bay Area.

The Bay Area comprises Hong Kong, Macao and nine cities in Guangdong, including Guangzhou, Shenzhen, Zhuhai, Jiangmen and Zhaoqing.

"The completion of the bridge will significantly change the regional transportation structure on the western bank of the Pearl River," said Chen Guanghan, chief expert of the Institute of Guangdong, Hong Kong and Macao Development Studies at Sun Yat-sen University.

He noted that Hong Kong will have greater influence in the western cities of Guangdong, promoting the development of local economies.

He suggested authorities on both sides should take innovative measures on boundary control arrangements so the benefits of the bridge can have full play.

The industrial environment would improve as exchanges become more frequent and convenient, he said.

The project is a platform not only for regional development but also for international cooperation.

Hans de Wit, managing director and chief tunnel expert of The Netherlands-based Tunnel Engineering Consultants, believes the bridge is a "magnificent project". His team has been working together with the bridge management bureau since 2008.

He said at the ceremony that the role of the underwater tunnel in the bridge complex exceeded that of all other tunnel and man-made island projects.

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