| Japan | Construction 1992–2006; in operation since 2006

What was done

Following repeated severe flooding from typhoons and heavy rainfall in the low-lying areas north of Tokyo, Japan's Ministry of Land, Infrastructure, Transport and Tourism built the Metropolitan Area Outer Underground Discharge Channel – widely known as G-Cans – the world's largest underground flood-water diversion facility. The system consists of five concrete containment silos, each 65 metres deep and 32 metres in diameter, connected by 6.4 kilometres of tunnels running 50 metres below ground, feeding into a vast pressure-adjusting water tank supported by 59 reinforced concrete pillars. During heavy rainfall, water from several rivers north of Tokyo is diverted into the silos and channelled through the tunnel network to the central tank, from which 78 pumps discharge up to 200 cubic metres of water per second into the larger Edo River, which can safely carry the load onward. The facility is a large-scale, capital-intensive grey-infrastructure response, in contrast to the blue-green, surface-based approaches used in Copenhagen and Rotterdam – reflecting Tokyo's extreme spatial constraints and the scale of flood risk it manages.

Key Outcomes

• Used roughly seven times a year on average, and has been activated more than 100 times since 2006, reducing flood damage in the protected area by an estimated two-thirds in terms of homes and area affected.
• In August 2008, during the highest recorded inflow, the system diverted approximately 11.7 million cubic metres of floodwater that would otherwise have inundated surrounding communities.
• Demonstrated the long-term viability of large-scale underground flood infrastructure as a complement to surface-based adaptation measures in dense, land-constrained cities.
• Now also functions as a public engagement and education asset: guided tours of the cavernous central tank – nicknamed the "Underground Temple" – attract visitors and raise public awareness of flood risk and infrastructure investment.
• Has informed flood-tunnel projects in other space-constrained cities, including Kuala Lumpur's SMART Tunnel, and is referenced internationally as a model for combining large-scale engineering with long-term climate resilience planning.

More information:

Metropolitan Area Outer Underground Discharge Channel

How Tokyo Built World's Largest Underground Water Tank For Flood Protection