Tokyo Expands Underground ‘Cathedral’ Complex to Combat Climate Change Rains, Reports Reuters

By Issei Kato and Tom Bateman

KASUKABE, Japan – Early on August 30, just after 5 a.m., a large underground chamber known as the "cathedral" was flooded as heavy rain swept across the Tokyo region, triggered by Typhoon Shanshan approximately 600 kilometers (373 miles) away in southwest Japan.

The cathedral, part of an expansive network of tunnels, successfully managed the excess water and prevented flooding in the vulnerable river basin of the metropolis. However, due to the increasing severity of weather events linked to global warming, authorities recognize that significant upgrades to the system are necessary.

"As temperatures increase, the quantity of water vapor in the atmosphere rises, resulting in heavier rainfall," explained Seita Emori, a University of Tokyo professor and member of a climate science group awarded the Nobel Prize in 2007. "We expect to see unprecedented rainfall amounts as temperatures continue to rise."

Japan faces a multitude of natural disasters, ranging from earthquakes and volcanic eruptions to typhoons and landslides. Similar to many regions globally, the country is grappling with extreme weather patterns exacerbated by climate change.

This summer marked the warmest on record since data collection began in 1898, while parts of northern Japan experienced devastating flooding from record rainfall in July. In Tokyo, sudden and intense storms known as "guerrilla" showers have become increasingly frequent.

The Metropolitan Outer Area Underground Discharge Channel, often called the cathedral complex, took 13 years and approximately 230 billion yen (about $1.63 billion) to construct. Since it became operational in 2006, it has reportedly prevented over 150 billion yen in flood damages, according to estimates by the land ministry.

Besides its engineering marvel, the complex is also a popular tourist destination and filming site, featuring a cavernous space capable of holding water equivalent to nearly 100 Olympic-sized swimming pools.

Inside, the chamber is supported by 59 colossal pillars, each weighing 500 tonnes (551 tons) and standing 18 meters (59 feet) tall. During flooding events, excess water from nearby rivers flows through 6.3 kilometers of massive underground tunnels before accumulating in the reservoir.

Visiting the bottom of the chamber offers an unusual experience; it’s considerably cooler in summer and warmer in winter compared to the surface. Clouds of mist hover around the tops of the pillars, creating an ethereal atmosphere.

The dimly lit interior, enhanced by natural light filtering through the ceiling openings, and the towering pillars evoke the feeling of an ancient temple, earning it various monikers like "the cathedral" or "the shrine."

The enormous No. 1 shaft is deep and wide enough to comfortably contain the Statue of Liberty.

The complex has been activated four times in June alone, surpassing the total number of activations from the previous year. During Typhoon Shanshan, it captured enough water to fill the Tokyo Dome baseball stadium nearly four times, efficiently channeling it into the Edogawa River and out to sea.

Yoshio Miyazaki, a land ministry official overseeing the facility, noted, "We are witnessing a worrying trend of intense rainfall occurring all at once, which we term guerrilla downpours. Without this facility, the levels of the Nakagawa River and its tributaries could rise to dangerous heights, potentially leading to severe flooding and loss of life."

Despite its effectiveness, the system could not prevent flooding that affected over 4,000 homes in the river basin during heavy typhoon rains in June 2023. These floods prompted a new seven-year, 37.3 billion yen initiative to enhance levees and drainage in the region.

Moreover, in central Tokyo, a major project is underway that will connect channels to manage overflow from the Shirako and Kanda rivers. This project, set for completion in 2027, aims to carry floodwater approximately 13 kilometers underground to Tokyo Bay.

Tokyo’s sewer system was designed for rainfall of up to 75 mm per hour; however, localized storms are now delivering as much as 100 mm, exceeding the system’s capacity, according to Shun Otomo, a construction site manager for the project.

"During a short downpour in the Kanda River basin, we can utilize watershed capacity from areas that are not experiencing rain," Otomo explained. "We believe this approach will be effective against intense downpours."

Reporting by Issei Kato and Tom Bateman; writing by Rocky Swift in Tokyo; editing by Miral Fahmy