Metropolitan area outer underground discharge channel facility

The world's largest underground flood-discharge system, nicknamed G-Cans, beneath National Route 16 in Kasukabe, about an hour north of Tokyo. Five vertical silos 70 metres deep and 30 metres wide intercept overflow from smaller rivers and channel it via a 6.3-kilometre tunnel into the larger Edogawa River. The signature space is the central pressure-relief tank, a 177-metre concrete hall with 59 eighteen-metre columns that reads like an underground cathedral and frequently doubles as a film location for kaiju and tokusatsu productions. Tours run only by booking through the Ryukyukan visitor centre, in Japanese and English. During peak rainy season the hall is sealed and flooded.