An engineer introduces construction of artificial islands for the HZMB. [Photo provided to China Daily]

5. Length, tunnel and artificial islands

HZMB is a 55-kilometer-long bridge-island-tunnel complex across the Pearl River Estuary. It is the world's longest sea crossing. The bridge consists of a 6.7-km undersea tunnel, which is the country's first offshore immersed tunnel and the world's longest immersed tunnel for road traffic. It is connected to the bridge through two offshore artificial islands.

The tunnel is built to reserve room for a planned shipping channel for passenger liners with a displacement of 300,000 tons. As the planned location is close to the Hong Kong International Airport, where there is height limitation in the airspace, a tunnel must be built subsea instead of a bridge with tall towers.