The Tokyo Bay Aqua-Line (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.
At the bridge-tunnel crossover point, there is an artificial island called Umihotaru (Umi-hotaru, literally meaning "sea firefly") with a rest area consisting of restaurants, shops and amusement facilities. Air is supplied to the tunnel by a distinctive tower in the middle of the tunnel, called the Kaze no Tō ("the tower of wind"), which uses the bay's almost-constant winds as a power source.
The Tokyo Bay Aqua-Line reduced the drive between Chiba and Kanagawa, two important industrial regions, from 90 to 15 minutes, and also contributed to cut the travel time to the sea leisure area in the southern part of the Bōsō Peninsula from Tokyo and Kanagawa. Before the tunnel opened, one had to drive a roundabout 100 km along the shores of Tokyo Bay and pass through downtown Tokyo.