It’s simply an engineering marvel, and unquestionably one of the most awe-inspiring wonder spots of Japan.

It has been described as “the concrete junkie’s equivalent to a dropper of heroin”.

And it lies hidden underneath the outskirts of metropolitan Tokyo.





what is it & why was it made?

Since before the dawn of human history the rivers running into Tokyo Bay flowed through nature and (in recent centuries) agricultural land. But gradually since the mid-1950s, this land has undergone massive urbanization – a tenfold increase from 5% in 1955 to over 50% today. This has been primarily at the expense of rice fields and other agricultural land, land which used to absorb most of the rainfall in the area.

Of course as concrete replaced earth, the drainage systems built were simply not enough to cope with the humungous volumes of water falling during the typhoon season which, with each passing year, had less soil available to soak into.

As a result, six major floods affected the area throughout the 1980s and 1990s, two of which caused major damage to over 30,000 homes.

Time to do something. Japanese ingenuity came to the rescue, soaked up 17 years and 2 billion dollars, and produced what has been described as “the concrete junkie’s equivalent to a dropper of heroin”.

The Water Discharge Tunnel on the Outskirts of the Metropolitan Area project is commonly referred to as “G-Cans” for short, from the Japanese gesuikanaru (下水カナル), meaning “drainage canal”. It is the biggest man-made drain in the world.





what it does & how it works

The aim was simple: to take all the excess rainwater from typhoons and floods away before it can cause any damage. The way it was implemented is also simple – they built a big drain. But the enormity and engineering complexity of the construction is staggering.





A series of 5 giant sinkholes has been dug into the ground, each designed to catch the runoff and overflow from nearby rivers. Each sinkhole looks something like the exit tunnel from the subterranean rocket base of a 1960s 007 James Bond villain. The sinks range up to 74m in height and 32m diameter. That’s big enough to house a Space Shuttle or one and a half Statues of Liberty with plenty of room to spare.





During heavy rains and typhoons, tons and tons of flood and rainwater pours down these sinkholes and falls into the next engineering marvel – a giant concrete pipeline acting as an “underground river” which transports the water more than 6.3km through a tunnel 50m underneath the river basin.





Finally the water reaches what is known as the “Underground Parthenon”, an incredible subterranean reservoir almost 180m long, 80m wide and 18m high. From here modified jet engine turbines pump the water out into the adjacent Edogawa (Edo River) at the phenomenal rate of 200 tonnes per second through sluiceways large enough to comfortably fit entire carriages of metro trains.

All in all, the Water Discharge Tunnel on the Outskirts of the Metropolitan Area is estimated to reduce the potential flood impact on buildings, homes and urban areas by over 80%. Tested many times during high rainfalls and tsunamis over the last few years since completion, it has clearly proven its value. Levels of rainfall that previously devastated thousands of homes now affect less than a hundred.





visiting G-Cans – the tour

Naturally for such an awesome spectacle, the Water Discharge Tunnel on the Outskirts of the Metropolitan Area has has attracted great tourist interest. Visitor numbers are limited for the regular tours (in Japanese only) and advance bookings are essential.

I visited the stunning construction last year, and now I take you step-by-step through the experience.

(click all images to enlarge)





Upon arrival at the site one registers in the visitor centre, where a wide variety of G-Cans related displays are on show. At the designated time a tour guide arrives with clever personal speaker system to introduce the tour.





First we’re shown an introductory video describing the area’s history, the reasons for the project’s existence and a G-Cans overview.



Then we gather around a large satellite image on the floor showing the river basin from the mountains all the way to central Tokyo and Tokyo Bay. An explanation of the topography of the area is given.





Note how much of the landscape is urbanised – all the grey stuff.

Just 50 years ago almost everything would have been green.





Next, visitors are shown the G-Cans project in graphics, videos and miniaturized models of the project’s main features.





Real water flows through the models as the tour guide explains.



Finally it’s off to see the real thing.

First the project Control Room with a row of 20 video screens.





Then we head off to the roof of the visitor centre, via a passageway lined with kids’ drawings of G-Cans inspired by their visits.





The Edo River into which up to 200 tonnes of water per second is pumped though huge channels under these paths.





Tokyo Sky Tree visible on the horizon.





The Underground Parthenon lies underneath a football field!





The chamber is entered through a concrete box at the far end of the football field.



(click images to enlarge)

Descending a few flights of stairs, we get our first glimpse…

…and arriving at floor level the view is, in a word, awesome.

People start snapping photos immediately we hit the floor. Note the rope lying across the concrete, beyond which one unfortunately isn’t allowed to go.





People are dwarfed by the behemoth concrete pillars. These images were taken looking across the space, which is less than half of its length.





The place is truly colossal in scale.





At the end of the cavern is a gigantic vertical shaft through which the water flows up from the subterranean tunnel following torrential typhoon rains and flooding.



After what feels like a very short time the group is called together once more to head up the stairs and back to reality.





G-Cans on film and TV

The monolithic pillared interior of the reservoir resembles something out of science-fiction and therefore has featured in many advertising, film and TV projects.

Displays on the walls of the visitor centre document many of the film projects that have used the space and are signed by famous actors and TV talents. The majority feature superheroes of course.

(click images to enlarge)

Producer/singer/songwriter Pharrell Williams recently visited G-Cans in filming the “Tokyo Rising” mini documentary series, which gives an insight into many of the creative individuals, locations and artists who have inspired his work.

Click the image above or click here to watch the video.





G-Cans links and more information

G-Cans information pack

Visitors to G-Cans can collect a 12-page booklet packed with images, statistics and information on the project.

Click here to download an English version.

G-Cans official site

Link to official site (in Japanese)

100 views of Mt. Fuji

G-Cans makes the cut as one of the best viewing spots for Mt. Fuji, according to the Kanto Regional Development Bureau.

They produced a brochure displaying 233 viewing spots from 128 locations, of which G-Cans is number 27.

Click to download the short version (Japanese only)

Click for the full version with detailed information (Japanese only)













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Posted in construction, japan, wonder stuff

Tags: concrete, japan, wonder