The Tokyo Bay Aqua-Line connects Kawasaki City of Kanagawa and Kisarazu City of Chiba by a 9.5km long underground tunnel and a 4.4km long bridge, having a total length of 15.1km. This project lasted for 30 years since the start of the planning in 1966 and used the latest technology. The new passage opened on December 18, 1997.The giant tunnel digging machine with a 14.14m caliber, “shield machine,” dug 160m a day in 28m of maximum water depth and 16m of overhead dirt, withstanding a 6kg per square centimeter water pressure. All of the construction such as the conveyance and assembling of the tunnel-supporting segments, including the digging management of the shield machine, was automated, and measuring technology of the highest precision was used. As for the joints where the docking of the shielding machine coming Kawasaki occurs, an astonishing precision of only a 4-11 millimeter error was achieved.

The symbol of the Aqua-Line is the resting facility “UMI-HOTARU” which floats above a water depth of 26m and at a distance of 5km from the off ramp at Kisarazu. It is a man-made island with the upper(overground) part being 100 m in width and 650 m in length, and the bottom(underground) part being 170 m in width and 750 m in length.　The 360 degree scenery of the sea spreads out, and the panoramic view of the city buildings beyond the water is becoming a popular sightseeing spot.

Obayashi Corporation was in charge of constructing the west side of the “UMI-HOTARU”(Kisarazu Man-made Island), and of the two tunnels, the one extending south to the Kawasaki Man-made Island, and also the buildings on UMI-HOTARU.