Construction work on a tunnel-type underground regulating reservoir is seen in Tokyo’s Nakano Ward, July 24, 2023. Members of the media photograph the rear of a shield tunneling machine and the completed tunnel lining. (Mainichi/Seiho Akimaru)
TOKYO — Japan has been hit by a string of devastating rainstorms in recent years, including the latest record rainfall that caused extensive damage in Chiba Prefecture. Against this backdrop, the Tokyo Metropolitan Government is considering building an “underground river” to capture water spilling over the banks of swollen rivers.
In March, the metropolitan government established a planning review committee composed of experts and other members, launching full-scale discussions on the project.
The idea of building underground rivers in Tokyo first emerged in 1985, during Shunichi Suzuki’s tenure as governor. At the time, heavy rain repeatedly caused small and medium-sized rivers, including the Kanda, Shirako and Shakujii rivers, to overflow.
Suzuki commissioned the Tokyo metropolitan underground river planning study group to examine possible flood-control measures. The panel issued an interim report in 1986. The Mainichi Shimbun reported on the proposal at the time under the headline: “Giant underground river planned to protect central Tokyo from flooding.”
The plan called for approximately 39 kilometers of underground waterways beneath Tokyo’s Ring Roads No. 7 and No. 8. The total cost was estimated at 1.1 trillion yen (some $6.9 billion at current rates), but the momentum behind the planned project gradually faded.
A panel of experts discusses the technical feasibility of the Ring Road No. 7 underground river project at the Tokyo Metropolitan Government building, July 14, 2026. (Mainichi/Kazuo Yanagisawa)
The concept returned to the spotlight in 2021, during Gov. Yuriko Koike’s second term. The construction of an underground river was explicitly included in the metropolitan government’s long-term “future Tokyo strategy,” and Koike also discussed the proposal in the Tokyo Metropolitan Assembly. The project was revived as a potential response to increasingly severe rainstorms and flooding.
Under the plan now being considered, two existing tunnel-type flood-control reservoirs would be connected: the 3.2-kilometer Shirako River underground regulating reservoir and the 4.5-kilometer Kanda River-Ring Road No. 7 underground regulating reservoir.
The two facilities would be linked by the planned 5.4-kilometer Ring Road No. 7 underground wide-area regulating reservoir. Additional facilities, including a regulating reservoir in the Meguro River basin, would then be constructed farther south, creating an underground river approximately 30 kilometers long. Water collected by the system would be discharged into Tokyo Bay.
Existing underground regulating reservoirs have helped reduce flood risks, but they alone are not sufficient.
A completed section of the Neyagawa southern underground river is seen in the city of Higashiosaka, Osaka Prefecture, in this photo provided by the Osaka Prefectural Government.
In June this year, Tokyo was struck by heavy rain caused by Typhoon Jangmi — the sixth typhoon of the year — and a seasonal rain front. Level 4 Flooding Urgent Warnings were issued for four rivers, including the Kanda River.
According to the metropolitan government, the Kanda River-Ring Road No. 7 underground regulating reservoir took in approximately 237,100 cubic meters of water during the storm. That was equivalent to 44% of its total capacity. The facility is estimated to have kept the river’s water level about 60 centimeters lower than it otherwise would have been.
Regulating reservoirs, however, have limited storage capacity. After a storm, the water they hold must also be released back into rivers. An underground river extending all the way to Tokyo Bay would instead be able to continuously channel excess water toward the bay, allowing the system to handle vastly greater volumes of floodwater. It is therefore expected to help reduce the damage caused by increasingly severe rainstorms.
In a 2025 interview with the Mainichi Shimbun, Koike expressed her enthusiasm for the project.
“One of the great achievements of the Tokugawa period (or Edo period between 1603-1867) was the large-scale diversion of the Arakawa River westward,” she said. “This latest project would be a monumental undertaking to create an entirely new river in the Reiwa era (2019-). I want this project to make Tokyo, the nation’s capital, more resilient and protect it.”
Osaka Prefecture is also building underground rivers in the city of Osaka, although the full network is not expected to be completed for another 50 years or so.
The Neyagawa River basin has repeatedly suffered flood damage in the past. In response, the Osaka Prefectural Government has been constructing two underground river systems since the 1990s: a 14.3-kilometer northern system and a 13.4-kilometer southern system.
Both are designed to connect small and medium-sized rivers and divert water from overflowing rivers into other waterways. As of 2022, the total project cost was estimated at approximately 442.6 billion yen (around $2.78 billion). Completed sections are already being used to temporarily store stormwater and have helped reduce flood risks.
The Tokyo metropolitan area outer underground discharge channel in Kasukabe, Saitama Prefecture, has also proved effective. The 6.3-kilometer facility became fully operational in 2006.
During a typhoon in August 2024, it collected 8.53 million cubic meters of water. Estimates indicate that the channel reduced flood damage in its basin by a total of 162.6 billion yen (approx. $1.02 billion at current rates) between June 2002 and June 2023.
Building an underground river, however, comes with an enormous price tag.
According to the Tokyo Metropolitan Government’s Bureau of Construction, the Kanda River-Ring Road No. 7 underground regulating reservoir cost approximately 100 billion yen (about $629 million), while the Shirako River underground regulating reservoir cost some 32 billion yen (roughly $201 million). The planned Ring Road No. 7 wide-area underground regulating reservoir is projected to cost approximately 135.1 billion yen (around $850 million).
Extending the system all the way to Tokyo Bay would require several hundred billion yen more.
Even so, Tadashi Yamada, a professor at Chuo University’s Research and Development Initiative who chairs Tokyo’s planning review committee, believes the project would be worthwhile.
“As flood disasters grow in scale around the world, underground rivers offer an effective solution,” he said. “They require enormous construction costs and many years to complete, but if we begin now, we should be able to protect future generations from flooding.”
(Japanese original by Kazuo Yanagisawa, Tokyo City News Department)