Tokyo builds an underground fortress against floods

Although Tokyo might seem like the antithesis of the "sponge city" concept, the Japanese capital is largely unaffected by floods and downpours. This is thanks to an engineering marvel known as the G-Cans, a colossal system of tunnels and reservoirs built beneath the city. Thanks to 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, and floodwaters rise in the rivers above the metropolis.

To protect Tokyo from extreme weather events, whose frequency increases with the progression of the climate crisis, construction of the great sewer system known worldwide by its English name, the Metropolitan Area Outer Underground Discharge Channel, or more briefly, G-Cans, began in the mid-90s.

Its purpose is to capture the floodwaters, channel them under the 37-million-strong metropolis, and then safely release the excess water below Tokyo into the Edo River, through which the waters flow into Tokyo Bay.

The construction work was completed in 2006, resulting in the world's largest sewer system, impressive both in size and efficiency. Its pumps can channel 200,000 litres of water per second. That's akin to capturing and pumping almost all the water from the Vistula River at the level of Warsaw (for the average water level, about 250,000 litres per second).

To make this possible, the water is captured by five silos with a height of 65 metres and a diameter of 32 metres – each of them so large that it could easily hold, for example, the American Statue of Liberty. The silos are connected by tunnels running 50 metres underground with a length of almost 6 kilometres.

They lead to the main reservoir – a gigantic underground chamber, called the "temple", 177 metres long, 78 metres wide, and 25 metres high. The pump system expels water from this chamber to a location that is safe for the metropolis.

For almost 20 years, the Japanese sewer system has repeatedly proven its effectiveness, protecting the city from floods. According to Japanese estimates, since G-Cans was put into operation, flood damage has been reduced by 90%. 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.