The 6,000-tonne wedge, the final piece of an undersea tunnel of the Hong Kong–Zhuhai–Macao Bridge (HZMB), was successfully installed on May 2, according to media reports.

A gigantic crane, which was transformed from a tanker, hoists a 6,000-ton key structure of the world's longest cross-sea bridge linking Hong Kong, Zhuhai and Macao, May 2, 2017. [Liu Dawei / Xinhua]

The wedge, 12 meters long, and weighing more than 25 Airbus A380 jets, was lowered to connect the tubes which will form the tunnel section of the bridge.

The tunnel, which is over 5.6-kilometer long, is a crucial and the final part of the construction of an overall 6.7-km underwater tunnel on the seabed that constitutes the bridge.

Once completed, the HZMB will have a total length of about 55 km, including a 6.7-km underwater tunnel and a 23-km bridge over the sea. It will connect Zhuhai in Guangdong Province with Hong Kong and Macao.

The construction of the underwater tunnel is reported to be a difficult part of the HZMB, the longest cross-sea bridge in the world.

Originally, instead of a tunnel, a bridge across the Lingdingyang waters was planned. It would have to be built over 80 meters to allow ships to pass, which also meant that bridge towers of at least 200 meters had to be constructed, reports CCTV News.

But a nearby Hong Kong airport requires that all buildings have to be lower than 88 meters, thus a tunnel was constructed instead.

One end of the bridge is close to the Lingdingyang Channel, one of the busiest trade routes in the world.

The undersea tunnel was also built to help the ships sail without hindering the busy water channel of the Pearl River estuary, according to media reports.

The impact of the bridge on marine life in the delta was also taken into account when its design was developed.

A gigantic crane, which was transformed from a tanker, installs a 6,000-ton key structure of the world's longest cross-sea bridge linking Hong Kong, Zhuhai and Macao, May 2, 2017. [Liu Dawei / Xinhua]

The water resistance ratio must be limited to 10% to prevent accumulation of sediment at the Lingdingyang Channel, otherwise it will impose serious impact on the Pearl River flow path and flood-discharge capacity, according to a government report.

When opened to traffic, the bridge will cut travel time from Hong Kong to Macao and Zhuhai from three hours to 30 minutes on the road.

The construction began in December 2009 at Zhuhai. The bridge is expected to open in December this year.