The Tokyo Bay Aqua-Line (東京湾アクアライン, Tōkyō-wan Akua-rain^?), also known as the Trans-Tokyo Bay Highway, is a bridge–tunnel combination across Tokyo Bay in Japan. It connects the city of Kawasaki in Kanagawa Prefecture with the city of Kisarazu in Chiba Prefecture, and forms part of National Route 409. With an overall length of 14 km, it includes a 4.4 km bridge and 9.6 km tunnel underneath the bay—the fourth-longest underwater tunnel in the world.