Behold the Cross Harbour Tunnel – that which links Hong Kong island and Kowloon. There are three such tunnels on the west, east and center of Hong Kong island. Prior to these tunnels, motor traffic between Hong Kong island and Kowloon had to cross the harbour via ferry.
Little note: vehicular ferries are still in operation as these tunnels are meant for vehicles of a certain size only. Larger trailers and transports carrying hazardous material (say, petroleum) aren’t allowed into the harbour crossing tunnels.