cross-harbor tunnel

this tunnel connects Kowloon with Hong Kong Island.

The Cross-Harbour Tunnel (abbr. CHT or XHT; (Chinese: 香港海底隧道, or informally 紅磡海底隧道, 紅隧) is the first tunnel in Hong Kong built under water. Since its construction, its traffic has increased, becoming one of the most congested roads (mainly towards Hong Kong Island direction) in Hong Kong and the world.

Constructed by a private company and operated under a 30-year franchise, the 1.8 km-long tunnel crossing opened in 1972, providing the first road link between Kowloon and Hong Kong Island; Cross-harbour vehicular traffic depended on ferries before the tunnel was built.

source: wikipedia