The £8.9bn tunnel that’s one of the world’s longest and links two very busy cities

The drive between two of major industrial areas used to take around an hour and a half, as it required traversing a 62-mile stretch along Tokyo Bay and through central Tokyo.

Today, stretching across the bay, the 15-mile Bay Aqua Line cut travel times to just 15 minutes. 

Connecting the city of Kawasaki in the Kanagawa Prefecture with Kisarazu in the Chiba Prefecture, this crossing made up of a bridge-tunnel combination forms part of National Route 409, which stretches for an incredible 74 miles across the country. 

One of the last major Japanese of the 20th century, it includes a three-mile bridge and a six-mile tunnel beneath the bay – earning it the title of the fourth-longest underwater tunnel in the world.

Also known as the Trans-Tokyo Bay Expressway, the roadway was built at the cost of 1.44 trillion Japanese yen (£8.9 billion) and opened in December 1997, following an incredible 23 years of planning and nine years of construction. 

A unique tower stands above the middle of the tunnel, supplying air to the tunnel. Its ventilation system is powered by Tokyo Bay’s almost constant winds. 

The six-mile , known as the Aqua Tunnel, sits an impressive 196 feet below sea level at its deepest point and has a diameter of 45 feet. 

Meanwhile, the three-mile Aqua Bridge has the longest span, 787 feet. A recent LED modification on the bridge has also reduced power consumption and by 40%.

The project also required careful consideration of environmental factors, including a soft seabed, high winds, tides and potential earthquakes. Therefore, engineers had to develop new technologies and construction methods to overcome these challenges, earning the nickname “Apollo project of civil engineering.”

An artificial island, Umihotaru or “sea firefly”, marks the transition from the bridge to the tunnel sections and hosts restaurants, shops and amusement facilities for those needing a rest break. 

Umihotaru consists of a five-story structure, with the three lower floors reserved for parking. From the observation deck, visitors can enjoy panoramic views of Tokyo Bay. On a clear day, you can see the iconic Tokyo Skytree, the skyline of Japan’s largest port city, Yokohana, and even in the distance. 

Tokyo Bay Aqua Line has slashed travel time between Haneda Airport and Kisarazu by around 45 minutes and has also facilitated easier access to the Boso Peninsula, boosting in the area which boasts impressive temples with stone statues and picturesque white sand beaches. 

At the time of opening, the roadway had the highest toll fee in Japan, with a one-way trip costing 5,050 yen (£26.70) per kilometre. 

The main aim of the Aqua-Line was to redirect vehicles away from central Tokyo. However, the expensive toll for the crossing has resulted in only a limited reduction in traffic.