We joined a tour organized by Edogawa River Office (江戸川河川事務所) whose mission is to prevent disastrous flooding in the Greater Tokyo Metropolitan area.
The northeastern part of Tokyo cosmopolis is notorious for inundation caused by several rivers running very slowly in a relatively flat area with almost no gradient. It used to be a swamp in the past.
The idea of preventing from inundation is, during a typhoon attack or in rainy season, to drain the overwhelming water from several rivers by pooling the excess amount of water in the underground surge tank (首都圏外郭放水路) temporarily and spit it out to the bigger river (Edo river).
Here is the official video in Japanese.
The big surge tank is called "Underground temple" in its outlook. When we were in the tank, there was a movie filming going on.
The beauty of the tour is it's free of charge and you get a postcard when departing.
Interested in the tour?
Then, click [ENGLISH: Summary of the tour].
720 kamikanasaki, Kasukabe, Saitama, 322-0111
Have a wonderful weekend.