Tokyo Bay Aqua-Line / Umihotaru

A 15.1 km sea-crossing expressway linking Kawasaki and Kisarazu across Tokyo Bay, with a 9.5 km undersea tunnel (Aqua-Tunnel) and a 4.4 km overwater bridge (Aqua-Bridge). The midpoint artificial island, Umihotaru, offers 360-degree views of the bay. Opened December 1997.

A Highway Under the Sea and Over the Water

Kawasaki in Kanagawa and Kisarazu in Chiba face each other across Tokyo Bay, but for most of modern history getting from one to the other meant a long detour by road or a ferry crossing. The Tokyo Bay Aqua-Line, which opened on 18 December 1997, changed that. Running 15.1 kilometres across the middle of the bay as part of National Route 409, the Aqua-Line crosses Tokyo Bay in two fundamentally different ways: underground and in open air.

The Kawasaki end enters the bay as the Aqua-Tunnel — a 9,607-metre undersea passage and the longest undersea road tunnel in Japan. Built using cutting-edge engineering methods of the day, the tunnel pushes through the floor of Tokyo Bay to emerge at a man-made island in the centre. On the Kisarazu side, the Aqua-Bridge carries traffic 4.4 kilometres across the open water, elevated above the bay surface so that driving it means watching the sea stretch to the horizon in both directions.

Umihotaru: An Island at the Centre of the Bay

Where tunnel meets bridge, in the middle of Tokyo Bay, stands the artificial island of Umihotaru — “sea firefly.” The island is 100 metres wide and 650 metres long, and its five-storey facility houses restaurants, shops, and rooftop promenades facing 360 degrees of open water. It is the only point along the Aqua-Line where drivers can stop, step out, and stand surrounded entirely by sea.

Construction began in May 1989 and took approximately ten years to complete, at a total cost of 1.44 trillion yen. The sea sections were built by the Tokyo Bay Crossing Road Company and the land approaches by the Japan Road Corporation. The engineering challenges required dozens of new techniques and methods developed specifically for the project — earning it the nickname “the Apollo Programme of civil engineering.”

Summary

The Tokyo Bay Aqua-Line opened in December 1997 and runs 15.1 kilometres, with an undersea tunnel (approx. 9.5 km) and an overwater bridge (approx. 4.4 km). The midpoint artificial island of Umihotaru — 100 m wide, 650 m long, five storeys — sits at the precise centre of Tokyo Bay and offers a full 360-degree view of the water. Driving the Aqua-Line and stepping off at Umihotaru is one of the more quietly spectacular ways to experience Tokyo Bay at close range.

Related: Shuto Expressway Bayshore Route (B)

Looking for hotels near Kawasaki or Kisarazu? Rakuten Travel has options across the Tokyo Bay area.