Metropolitan Outer Area Underground Discharge Channel 首都圏外郭放水路
Explore Tokyo's depths at this underground marvel of modern engineering
Because low-lying Tokyo is vulnerable to flooding during the rainy and typhoon seasons, authorities built the world's largest underground flood diversion facility to mitigate any potential disasters. Now open to the public, tours of the center are free, but visitors are advised to enter at their own risk.
- Wandering the massive underground corridors and tunnels of the Kasukabe Underground Flood Protection Tank
- Learn about the building of the tank and disaster prevention at Ryukyukan Underground Exploration Museum
- Soaking up the tunnel's otherworldly atmosphere
How to Get There
The tank lies under Kasukabe, a town in Saitama Prefecture that's about 20 miles from central Tokyo and accessible by train.
The nearest station is Minami-Sakurai Station on the Tobu Noda Line. Take the Takasaki Line from Ueno Station or shinkansen from Tokyo Station to Omiya Station. From there, transfer to the Tobu Noda Line. From Minami-Sakurai Station, it's a seven-minute taxi ride.
A feat of modern engineering
The official name of the tank and tunnel system is the Metropolitan Outer Area Underground Discharge Channel. This concrete tunnel system running 50 meters deep and 6.5 kilometers long, it's also known as the G-Can Project or “The Underground Tunnel.” This feat of modern engineering was built from 1992 to 2006. About seven times a year, it diverts water from heavy rainstorms and keeps the streets of Tokyo from turning into gushing rivers.
Tunnels big enough to house the Statue of Liberty, five times over
This complex system consists of five massive collection silos connected by four miles of tunnels. Each silo is big enough to contain an entire Statue of Liberty. During heavy rainfall, water enters through the silos and runs along a central tunnel, before shooting through a surge tank, and being released into Tokyo Bay.
Into the abyss
When not diverting water into the bay, the Kasukabe Underground Flood Protection Tank is open to visitors. A trip down into its depths is free, but you need to book your trip about four weeks in advance. Visiting the tunnel is popular and visiting times are restricted.
Safety is paramount at the complex. Visitors are required to be accompanied by a Japanese speaker and must conduct safety drills and exercises before heading down into the tunnels. Sudden cancellations are not uncommon when the weather turns inclement.
Learn the ins and outs before you enter
Before reaching the tunnels, you pass through the Ryukyukan Underground Exploration Museum. Here, you learn about how the system was built and how it works. After that, you're taken to an area where you can walk around among the massive pillars and endless tunnels.
Not of this earth
If you're into Japanese film, television dramas or gaming, you may recognize some of your surroundings. The site has been used as an otherworldly backdrop for movies and commercials, as well as a design base for video game levels.