The Metropolitan Area Outer Underground Discharge Channel (also called Gaikaku in Japanese) is just outside of Tokyo and was built to prevent floods in the area while heavy rain or typhoons. There are five huge underground tanks, connected via tunnel leading into this huge hall, which fills up when the rivers overflow.
This huge facility is located just north of Tokyo, in Saitama. This area is one big valley with a few rivers flowing through. With heavy rainfall, the whole valley and the area below would get flooded without this system.
We made a half day trip out of this. From the southern part of Tokyo, we had to get through Tokyo and head north, which all in all took a bit over two hours.
It's still a short distance from the nearest train station, so we went a bit earlier and had a nice walk through some rice fields (we took the bus on the way back though).
With our tour reservation (you can make reservations here, but tours are only in Japanese), we started the tour in a group of about 20 people. There are three different tours which explore different aspects of the facility. After getting an explanation how the flood prevention works, we started to walk around.
This facility consists of five huge cylindric tanks, which are scattered throughout the valley. If the rivers flood, excess water will flow into these tanks, which are connected by a tunnel. These tanks can hold an enormous amount of water, which will gradually flow into the pressure-adjusting water tank at the end of the tunnel.
Here, the water will be slowly pumped into another river once water levels have sunken.
First, we got to see the control room, where water levels are observed continuously. We then got to see the turbines which pump the water into the river. At the end, we descended a lot of stairs entering the pressure adjusting tank.
We visited just a few days after a typhoon, so there where still puddles of water left in the tank. Imagine this huge tank filled with water!
When the tank is being used, you won't be able to enter the tank, so be advised when making a reservation.
After 20 minutes of free time in the tank (photo time!), we had to walk the stairs up again the tour ended.
It is impressive just how much water this infrastructure can handle, definitely a great experience to visit. You have to choose your tour and make reservations in advance. Also it's a bit outside of Tokyo, so plan your travel well, since you don't want to be late for your tour.
The tour is in Japanese and the facility wants at least one Japanese speaking person in your group (since you have to at least translate the safety measures). Wheelchair access is limited and there are some stairs if you want to see the cool parts.