A graphic shows how the Hong Kong-Zhuhai-Macao Bridge connects the three cities. [Photo/chinadaily.com.cn]

World's longest cross-sea bridge moved one step closer to completion as the construction of the main structure was completed on Tuesday, reported China Central Television.

With the final expansion joint laid down, the 55-km Hong Kong-Zhuhai-Macao Bridge marked a milestone on Tuesday morning.

Hailed by The Guardian newspaper as "one of the seven modern wonders in the world", it is the world's longest bridge under construction. The bridge is comprised of the offshore bridge and tunnel, boundary crossing facilities in Hong Kong, Zhuhai and Macao, and routes connecting the three cities.

The main structure of the bridge is made up of the 6.7-km underwater tunnel and a bridge spanning 22.9 km. Artificial islands are built at both ends of the tunnel to allow the passage of large ships.

Next, a full-scale effort can begin on the bridge deck pavement and the final-stage construction of the artificial islands and the tunnel, said the Hong Kong-Zhuhai-Macao Bridge Authority.

The construction of the bridge is a mega project. According to the authority, the amount of iron used in the main structure alone weighs around 400,000 metric tons, enough for about 60 Eiffel Towers. The deck pavement covers an area as large as that of 98 football fields.

The tunnel, more than 40 meters under the sea, is made of 33 sections of tube, each weighing 80,000 tons – or the weight of a large aircraft carrier. Eighty percent of the tunnel has been finished, said the authority.

In addition, 79 expansion joints are laid on the bridge deck to adapt to the changes in temperature and the shift caused by vehicles.

The bridge is also built to the highest quality standards – it can be used for 120 years, resist level-16 typhoon and earthquake of magnitude 8.0, and the strike of a ship with capacity of 300,000 tons.

The bridge, planned in 1983, is expected to be completed by the end of next year at the earliest. Once it is completed, the commuting times between Hong Kong and Zhuhai or Macao will be reduced from 3 1/2 hours to only 30 minutes.