On the surface, almost nothing indicates the huge underground reservoir from Kasubake to Saitama (north of Tokyo), the largest of its kind in the world.

As long as two football fields and supported by 500-ton pillars, the huge structure reaches a depth of almost 100 meters.

The installation allows the accumulation and redirection of excess water from rains, thus protecting one of the most populous metropolises on the planet.

The staff of the resort is always on the alert, especially during the rainy season, which generally manifests itself in Japan between the beginning of June and the end of October.

“We are in an area where torrential rains and even heavy rainfall can flood the streets and sink houses,” Nobuyuki Akiyama, head of infrastructure at Kasukabe, told AFP.

Completed in 2006 after decades of work at a cost of 230 billion yen (nearly 2 billion euros), the plant is used an average of seven times a year.

Excess water automatically reaches where it is pumped out of the main tank as it approaches maximum capacity, Akiyama explains.

The reservoir is connected to a 6.3-kilometer-long tunnel and equipped with a system that can discharge the equivalent of a 25-meter pool into the nearby Edogawa River every second.


According to Akiyama, the reservoir protects 90% of homes from flooding. In addition, according to official studies, the facility also saves 148 billion yen (1.2 billion euros), money that would be used for operations to return to normal after a disaster.

Japan’s flood control systems are among the best in the world, the AFP notes, noting that the Asian country has learned its lesson from the many natural disasters it has endured since World War II, such as the typhoon giant struck the Wakayama region (west) in 1959, killing more than 5,000 people.

This typhoon, the most difficult in Japan’s recent history, has made the Japanese understand the role of infrastructure in reducing risks.

In the Tokyo region alone, which is crossed by more than 100 rivers, there are ten other underground reservoirs such as the one at Kasukabe and three flood protection tunnels.

However, experts warn that more is needed. And that’s because the risks of natural disasters are growing against the background of climate change. For example, typhoons have multiplied by 1.5 in the last 40 years. Or in the past there was only one in a century.


Kei Yoshimura, a professor of meteorology at the University of Tokyo and a flood expert, is involved in developing an early warning system for areas most exposed to natural disasters. “It is clear that infrastructure works are not enough in the face of natural disasters,” he said.

On the other hand, the Japanese authorities are focusing on raising public awareness in order to follow the evacuation recommendations. In this sense, the Kasukabe tank receives visitors when it is not used for water discharge. Tonu Tamai, a 79-year-old pensioner, said he was impressed by the size of the facility. But “it’s just a defense. In the end, you only count on yourself, “he told AFP.