Tokyo expands underground 'cathedral' to avoid flooding caused by climate change

Increasingly intense storms due to climate change are forcing Japanese authorities to expand a network of flood tunnels beneath Tokyo.

Shortly after 5 a.m. on Aug. 30, water began to flood into a giant underground vault nicknamed the "Cathedral" in northern Tokyo. The torrent, captured on security cameras, was rainwater pouring down on the capital as Typhoon Shanshan made landfall 600 kilometers (375 miles) away in southwestern Japan, Reuters reported. The "Cathedral" and its vast network of tunnels have done their job of protecting the megacity's river basin from flooding. But as global warming brings more extreme weather, authorities need to upgrade the system.

"As temperatures rise, the amount of water vapor in the atmosphere increases, leading to heavier rainfall," said Seita Emori, a professor at the University of Tokyo and a member of the climate science team that won the Nobel Prize in 2007. "We predict unprecedented rainfall will fall as temperatures continue to rise in the future."

Pillars in the flood tunnel system under Tokyo. (Photo: CBS).

Japan is regularly hit by natural disasters, from earthquakes, volcanic eruptions, typhoons to landslides. The country is also facing unprecedented weather extremes due to global warming. This summer was the hottest since 1898, while record rainfall in the north caused catastrophic flooding in July. In Tokyo, violent and unpredictable storms known as "guerrillas" are becoming increasingly common.

The Cathedral Complex, officially called the Metropolis Subterranean Drainage Canal , took 13 years and $1.63 billion to build. Since it opened in 2006, the system has prevented more than $1.06 billion in flood damage. The cave-like complex can hold the equivalent of nearly 100 Olympic swimming pools. Inside the complex are 59 giant pillars, each weighing 551 tons and 18 meters high. When nearby rivers overflow, floodwaters are channeled through 6.3 kilometers of underground tunnels before converging in the reservoir.

Walking down six floors to the bottom of the vault is a unique experience. The system has a microclimate, much cooler than the ground in the summer and warmer in the winter. Mist clouds obscure the tops of the columns. The dark space is illuminated by natural light filtering through gaps in the ceiling. The towering columns are reminiscent of ancient structures, leading to the nickname "Cathedral." Vault No. 1 is deep and wide enough to fit the Statue of Liberty.

The system was activated four times in June, more than the total number of times it was used last year. During Typhoon Shanshan, the structure collected enough water to fill nearly four Tokyo Dome stadiums before safely discharging it into the Edogawa River and out to sea. 'Compared to previous years, there is a tendency for large amounts of rain to fall all at once ,' said Yoshio Miyazaki, a land ministry official in charge of the tunnel complex. 'If the system did not exist, the water level of the main Nakagawa River and its tributaries would be much higher, leading to flooding of houses and even loss of life.'

Even so, the system failed to prevent more than 4,000 homes in the river basin from being flooded after heavy rains in June 2023. That flooding prompted authorities to embark on a seven-year, $250 million project to reinforce levees and drains in the area. Closer to central Tokyo, another major project is underway to connect overflow channels from the Shirako and Kanda rivers. When completed in 2027, the system will channel floodwaters 13 kilometers underground into Tokyo Bay.

Tokyo's sewer network is designed to handle up to 75mm of rain per hour, but an increasing number of local storms can bring up to 100mm of rain, overloading the system, according to Shun Otomo, a construction site manager in Tokyo . "For example, if there is heavy rain in the Kanda River basin, we can take advantage of the infiltration capacity in the area where it doesn't rain ," he said.