The Tokyo Bay Aqua-Line is also called as the Trans-Tokyo Bay Highway. It connects the city of Kawasaki in KanagawaPrefecture with the city of Kisarazu in ChibaPrefecture, according to Wikipedia.
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.
The Seikan Tunnel is the deepest operational main-line rail tunnel in the world (though Guangzhou Metro Line 3's central tunnel has been longer since 2010) and the second longest after the Gotthard Base Tunnel in Switzerland. While the Seikan Tunnel is also the longest undersea tunnel in the world, the Channel Tunnel between the United Kingdom and France has a longer undersea portion