The Tokyo Metropolitan Area Outer Underground Discharge Channel (MAOUDC), simply known as Tokyo Flood Tunnels or ”G-can”, is a diversion floodwater facility and an engineering marvel. Due to its huge space, the tunnel is sometimes called an ”underground temple” or ”underground cathedral” despite not being a worship place.

The total cost of the Tokyo Flood Tunnels was about 3 billion dollars.

Due to its geography, Japan has suffered from many floods over the years. The typhoon that hit in 1991 was especially devastating for the country, so the Japanese officials accelerated their efforts and started to work on a project in 1993. With technology’s help, they designed a tunnel to minimize the damage caused by this natural disaster and it took 13 years to complete the whole project. This engineering marvel is a 6.4 km long system of tunnels and cylindrical chambers that save Tokyo and nearby areas from floods. It is now the world’s largest diversion floodwater facility.

The flood control system lies 50 meters below Kasukabe. It can clear up to 200 tons of water per second.

Tokyo’s Flood Tunnels connect five massive silos measuring 65m high and 32m wide to one massive tank which is the G-can. The giant reservoir is the system’s centerpiece measuring 25 meters high, 177 meters long, and 78 meters wide, and is supported by 59 pillars. About seven times a year, the overflowed water pours into the silos. The facility’s turbines send the excess water into the Edo River, which flows into Tokyo Bay and protects the city from floods.