Tokyo's underground marvel: How G‑Cans keep floods at bay

Although Tokyo might seem like the antithesis of the "sponge city" concept, the Japanese capital remains unbothered by floods and downpours. This resilience is thanks to an engineering marvel known as the G-Cans, a gigantic system of tunnels and reservoirs built beneath the city. Because of this system, Tokyo avoids flooding.

Heavy rains are an annual norm for Tokyo, which lies in the monsoon influence zone. At the end of summer, with the impact of typhoons, the city experiences intense rains each year, causing a flood wave to rise on the rivers above the metropolis.

To protect Tokyo from extreme weather events, whose frequency has increased with the progress of climate change, construction of the great sewer system began in the mid-90s. This system, known worldwide by its English name, the Metropolitan Area Outer Underground Discharge Channel, or more briefly, G-Cans, was designed to manage these floods.

The system's purpose is to capture the flood wave, channel it under the 37-million-strong metropolis, and safely release the excess water into the Edo River, allowing the wave to flow into Tokyo Bay.

Construction work was completed in 2006, resulting in the world's largest sewer system, impressive both in size and efficiency. Its pumps can channel 7,063 cubic feet of water per second.

To achieve this, five silos capture the water, each 213 feet high and 105 feet in diameter. These silos are so large that one could easily hold the American Statue of Liberty. The silos are connected by tunnels running 164 feet underground and stretching almost 4 miles long.

These tunnels lead to the main reservoir, a gigantic underground chamber called the "temple." This chamber is 581 feet long, 256 feet wide, and 82 feet high. The pump system expels water from this chamber to a safe location for the metropolis.

For almost twenty years, the Japanese sewer system has repeatedly proven its effectiveness, protecting Tokyo from floods. According to Japanese estimates, flood damage has been reduced by 90% since G-Cans was put into operation. Currently, when the metropolis is not threatened by rainfall, the impressive underground infrastructure serves as one of Tokyo’s tourist attractions and is open to visitors.