The Akuarain, as it's called in Japanese, is the longest underwater road tunnel in the world, connecting the cities of Kawasaki and Kisarazu on either side of Tokyo Bay. Planning for the route began in 1966, but it was not completed until December of 1997, by which point it had racked up a total cost of over $11 billion.

Before the Aqualine was completed, the only practical way to get from Tokyo and Yokohama to the Boso Peninsula was to take a ferry. Driving around the coast of Tokyo Bay took far too long. The Aqualine shaves off about an hour of driving time between Kawasaki and Kisarazu, as well as points south, making it easier for Tokyo residents and visitors to make day trips to the Boso region. It also makes it easier for residents of outer Chiba to get to Tokyo, as well as other points in Japan: the Kawasaki end of the route is adjacent to Tokyo International Airport.

The entire four-lane road is 15 km long, 9.5 km of which is underwater. The tunnel begins on the Kawasaki shore of Tokyo Bay and surfaces on an artificial island called Umihotaru, which has a rest area and parking garage. From there, a 4.4 km multi-span bridge completes the journey to Chiba prefecture. So essentially, to an untrained eye, the Aqualine appears to be a long pier that descends into the sea and ends on a tiny island with a building on it... a truly surreal sight.

Right above the midpoint of the tunnel is Kazenoto, a circular island about 200 m wide. Kazenoto holds up two conical black and white pylons, about 96 m high, which contain ventilation shafts for the main tunnel. The island is surrounded by a special shock-absorbing wall to protect against ship collisions (keep in mind that there are still many, many heavy cargo vessels trying to get to and from Tokyo).

The Aqualine charges a toll of ¥2,400 for cars, and up to ¥8,450 for large trucks, although these tolls are scheduled to become even steeper by the end of this decade.

Overall, the project is further proof that Japanese civil engineers are a force to be reckoned with (compare the Akashi Kaikyo Bridge, Kansai International Airport, and Seikan Tunnel).