In a country like Japan, where torrential rains and typhoons are recurring phenomena, water management becomes a vital necessity. In response to the persistent flooding problems in the Tokyo metropolitan area, the Japanese government undertook the construction of one of the world’s most impressive hydraulic infrastructures: the Metropolitan Area Outer Underground Discharge Channel (MAOUDC), known in Japanese as Shutoken Gaikaku Housuiro and popularly as G-Cans.

This colossal underground system, located in Saitama Prefecture, was designed to mitigate floods by diverting excess water from local rivers into the mighty Edo River. Its scale and complexity have led to it being popularly known as the underground temple, due to its immense pressure-regulating chamber, supported by imposing concrete columns. It is also the largest underground hydraulic infrastructure of its kind in the world.

The region where this drainage system is located has historically been prone to flooding. For centuries, the Tonegawa and Nakagawa rivers and their tributaries have wreaked havoc on surrounding communities. With the rapid urban growth of the 20th century, the risk of catastrophic damage increased significantly.

giant tunnels tokio floods
View of the colossal pressure regulation chamber. Credit: Kunitaka NIIDATE / Wikimedia Commons

In 1992, feasibility studies began for the construction of a large-scale underground drainage system. A year later, in 1993, construction of the channel began, taking more than a decade to complete. The first phase of the system became operational in 2002, and finally, in 2006, the entire infrastructure was put into operation.

The total cost of the project was estimated at approximately 230 billion yen, equivalent to more than 2 billion dollars. This investment has drastically reduced flood damage in the region, benefiting millions of residents in the Tokyo metropolitan area.

The MAOUDC spans a total of 6.3 kilometers and is located at a depth of approximately 164 feet (50 meters) underground. Its main function is to collect excess water from local rivers and channel it into the Edo River, thus preventing urban waterways from overflowing.

giant tunnels tokio floods
When not in use, the tunnels are open to the public. Credit: AMANO Jun-ichi / Wikimedia Commons

The system consists of the following key elements:

1. Intake Shafts

The MAOUDC features five enormous vertical shafts, designed to receive water from various tributaries. These shafts have diameters of up to 31.6 meters and depths reaching 74.5 meters. Each shaft is strategically located to intercept the flows of the Nakagawa, Kuramatsugawa, Ohgurokawa rivers, and other smaller streams.

2. Drainage Tunnels

An extensive 10-meter diameter tunnel connects the shafts to the pressure-regulating chamber. This tunnel was built using the slurry pressure shield method, an advanced underground excavation technique used in large-scale infrastructure projects.

3. Pressure-Regulating Chamber

This is, without a doubt, the system’s most emblematic component. Measuring 177 meters in length, 78 meters in width, and 18 meters in height, this gigantic hall is supported by 59 concrete columns, each weighing 500 tons. Its purpose is to reduce the water’s velocity before being released into the Edo River, preventing turbulence and erosion in the system.

4. Shōwa Pumping Station

Located at the system’s discharge point, this pumping station is responsible for transferring water from the pressure-regulating chamber into the Edo River. It is equipped with four high-capacity pumps powered by Mitsubishi MFT-8 turbine engines, capable of expelling up to 200 cubic meters per second.

Impact and Benefits

Since its activation, the MAOUDC has been used more than 130 times (as of 2021), with an average of 7 to 8 activations per year. Thanks to its operation, the number of floods in the region has significantly decreased, protecting densely populated areas and reducing the economic damage caused by these phenomena.

giant tunnels tokio floods
The size of the structure is colossal. Credit: Takashi Ota / Flickr

Beyond its functional importance, the system has also become a tourist attraction and even a cultural icon. Guided tours have been conducted in its impressive pressure-regulating chamber, attracting thousands of domestic and international visitors. Additionally, the structure has been used as a filming location for movies, television series, and music videos, reinforcing its image as an underground temple.

The Metropolitan Area Outer Underground Discharge Channel is a testament to modern engineering’s ability to tackle large-scale natural challenges. Its innovative design and impact on disaster reduction make it a masterpiece of hydraulic infrastructure.

With this project, Japan has demonstrated that planning, investment, and technology can come together in effective solutions for water management and the protection of future cities.

This article was first published on our Spanish Edition on March 19, 2025: Los gigantescos túneles que protegen a Tokio de las inundaciones a 50 metros de profundidad

SOURCES

John Sheer, Metropolitan Area Outer Underground Discharge Channel

G-Cans

Metropolitan Outer Area Underground Discharge Channel (Web oficial)

Jordy Meow, G-Cans: Le Temple Souterrain du Japan

Wikipedia, G-Cans

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