Exploring the G-cans: Japan’s Massive Underground Drain

Located 50 metres below the ground in Kasukabe, Saitama Prefecture, the Metropolitan Outer Area Underground Discharge Channel (首都圏外郭放水路) is a disaster-prevention facility built to safeguard against flooding in the Greater Tokyo Area. A bit of a tongue twister in either language, the facility is also unofficially known as the G-cans, which, for brevity, is what I will refer to it as for the remainder of this article.

G-cans main water tank, aka the underground temple

The G-cans is one of the largest underground drains in the world, with tunnels spanning 6.3 kilometres and an 18-metre high pressure adjusting tank that is 177 m long and 78 m wide.

The latter is what forms the main attraction for tours around the facility due to its resemblance to a cool-looking massive underground temple. Its dystopian appearance makes it a great location for shooting films, including the 2015 movie The Hunger Games: Mockingjay – Part 2.

G-cans main water tank, aka the underground temple

Overview of the G-cans

The role of the G-cans facility is basically to channel water from small rivers into the Edogawa River whenever they threaten to overflow. There are four shafts that connect to one or two smalls rivers from which water automatically flows when the water level rises to a particular level. The fifth shaft doesn’t connect to any river but to the pressure-adjusting tank. The shafts are all connected via tunnels.

G-cans main water tank, aka the underground temple

The pressure-adjusting tank, aka the “underground temple,” consists of 59 colossal pillars each weighing 500 tonnes. Water accumulates in this giant pool to reduce the momentum of the water flowing from the tunnels.

G-cans main water tank, aka the underground temple

On the other side of the tank is a drainage pump station, which is controlled from the central control room in the administrative building. Pumps work to send water into the drainage sluiceway, which then flows into the Edogawa River.

Tours of the G-cans

There are three types of tours available for booking at the facility. All participants must first sign in at Ryu-Q-Kan (龍Q館), the museum of the facility, at least 10 minutes before the tour on the day.

Ryu Q Kan
Ryu-Q-Kan (龍Q館)

Underground Temple Course

Cost: 1,000 yen
Duration: 55 minutes
Max participants: 50 people

This condensed tour takes you through the main water tank, where participants can also see the No. 1 shaft. You will need to descend down around 100 stairs to reach the tank. Once reassembled, staff will explain the mechanism of the underground channel before giving participants around 20 mins free time to take photos.

Posing in the G-cans underground temple
The traffic cones destroy the atmosphere a little.

Pump Facility Course

Cost: 2,500 yen
Duration: 100 minutes
Max participants: 20 people

The tour is for mech fans and those interested in the specific workings of the underground channel. Participants can see the pump room and gas turbines, in addition to the main tank. The outer drainage channel utilises four large centrifugal pumps and four gas turbines.

Shaft Course

Cost: 3,000 yen
Duration: 110 minutes
Max participants: 20 people

This is most expensive and comprehensive tour offered by the facility. In addition to the tour of the tank, participants also will be able to use the “catwalk” (a walkway for workers) and descend partway down the No. 1 shaft via stairs. The No. 1 shaft is around 70 m deep, with an internal diameter of 30 m. Wearing a helmet and harness is mandatory.

No. 1 shaft at the Metropolitan Outer Area Underground Discharge Channel
No. 1 shaft

How to book

You can make bookings on the official website, in English or Japanese, up to a day prior. Cancellations through the website can be made up to three days prior without any penalty. From two days prior, a 100% cancellation fee is charged.

Entrance to the main water tank
Entrance to the main water tank, aka underground temple

Getting to the G-cans

The closest station, located a 25-minute walk away, is Minami-Sakurai on the Tobu Urban Park Line. Public transport to the facility is scarce. There is a bus that runs very infrequently that will take you to the facility in 8 mins. You can check the timetable here. The fare is 150 yen, and you get off at the stop “龍Q館 (Ryu-Q-Kan).” Otherwise, your best option is to just walk the 2.3 kilometers from the station, or take a taxi if you don’t mind splurging.

Address: 埼玉県春日部市上金崎720
720 Kamikanasaki, Kasukabe-shi, Saitama Prefecture

Looking for more interesting things to do in Saitama Prefecture? Check out the newly-opened Tokorozawa Sakura Town, or, for something even more off-the-beaten-path, this abandoned convenience store.

2 thoughts on “Exploring the G-cans: Japan’s Massive Underground Drain

  1. Dear Doni,

    the Tonhalle Düsseldorf is one of the great concert halls in Germany. Right now we are preparing our new season magazine for 21/22 and are planning a photo spread on “Modern Treasuries” in it. The G-Cans in Tokyo will also feature in it, as a metaphor for an “empty treasure chamber”, so to speak. During our photo research, we came across your blog and would very much like to print the photo you took of the entrance to the Main Water Tank:

    (“exploring the g-cans” / the last picture at the bottom of the page / “How to book”)

    Could you tell us the conditions under which we would be allowed to print the photo in a size of approx. 10x15cm? Would you provide us with the picture in a printable high res quality?

    The magazine of our concert hall is a non-commercial product for our audience and is not sold. Here’s a link to the 20/21-issue: https://www.tonhalle.de/service/seite/magazin-und-jahresvorschau-oton/

    We would be very pleased to receive your feedback.


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