Inside Tokyo's massive underground flood control system

The Metropolitan Area Outer Underground Discharge Channel (MAOUDC) project, featuring giant underground water storage tanks, was completed after 13 years of construction, helping Tokyo reduce flood damage by 90%.

The main above-ground building of the Metropolitan Area Outer Underground Discharge Channel (MAOUDC) in Masukabe, northern Tokyo.

Tokyo, one of the world's largest metropolises, has a long history of flood control. This is due to its location on a plain surrounded by five river systems and 100 rivers.

In 1992, Tokyo began the MAOUDC project, with a total cost of 230 billion yen (2.2 billion USD). MAOUDC became operational in 2006, making it the world's largest flood control facility.

Diagram of MAOUDC in Tokyo. Graphic: New Economy.

This underground tunnel system draws water from small and medium rivers in northern Tokyo, funneling it into five massive underground cylindrical tanks. Each tank is 70 meters high, large enough to hold a space shuttle or the Statue of Liberty.

Water from the underground tanks then flows through a 6.3 kilometer tunnel. There, 78 high-capacity pumps discharge it into the Edo River at a rate of nearly 200 cubic meters per second.

An MAOUDC employee observes from the wall of underground tank number 1.

Inside underground tank number 1, which is 72.1 meters high, a system of stairs leads to the bottom for maintenance.

If water levels in the Edo River rise, MAOUDC reduces flow through a massive pressure control tank, as long as two football fields. This allows the pumps to regulate and push water into the river.

This massive control tank is known as "Pantheon" because it is supported by 59 large pillars, making it resemble a Greek temple. Each pillar weighs 500 tons.

MAOUDC employees walk inside the system's pressure control tank. The space inside the tank can hold water equivalent to 100 Olympic swimming pools.

"It's like a facility from a science fiction story," said Miki Inaoka, a disaster expert at JICA.

MAOUDC operates about 7 to 12 times a year, especially during Japan's rainy and typhoon season from June to late October.

"In this area, heavy rain, typhoons, or even daily rainfall can cause flooding," said Nobuyuki Akiyama, MAOUDC director. "This system has helped prevent 90% of infrastructure damage from floods in the area."

During typhoon Shanshan in 2024, the system stored enough water to fill four Tokyo Dome stadiums before safely pumping it into the Edo River and out to sea.

When not in operation, MAOUDC opens its doors to visitors to raise public awareness about the importance of disaster management.

Inside the MAOUDC control room.

A guide assists visitors entering the MAOUDC control tank.

Visitors tour inside the MAOUDC control tank.

To date, MAOUDC is part of a flood control network of over 28 regulating reservoirs across Tokyo, with seven other facilities under construction. When completed, the entire network can handle 100 millimeters of rainfall per hour, more than London's rainfall in two months.