The Hong Kong Zhuhai Macao Bridge is the longest fixed cross sea and high seas connection line in the world, connecting three major cities in the the Pearl River Delta. This 55 kilometer long bridge adopts a tunnel system, consisting of three cable-stayed bridges, a 5.6-kilometer-long underwater tunnel, and four artificial islands. The construction cost is 18.8 billion US dollars, and the design lifespan is 120 years.