At the far end of the regulating tank are four giant impellers, the largest of their kind in Japan, each powered (via an impressively large gear reducer) by a modified version of an aircraft gas turbine. At full output, even with all of the silencing systems, it must sound unsettling.
The massive drainage shafts take in water via overflow levees from the No. 18 Water Channel 第18号水路 (shaft two), Naka River 中川 and Kuramatsu River 倉松川 (shaft three), Koumatsu River 幸松川 (shaft four) and Otoshifurutone River 大落古利根川 (shaft five), which then flows to the first shaft and into the tank where the pumps regulate the water flow to keep the level within a safe operational range. It's expelled via six subway tube-sized sluiceways into the Edo River 江戸川, making its way into Tokyo Bay 東京湾 and keeping it safely out of the Naka River 中川 and Ayase River 綾瀬川 basin.
The system has proved its effectiveness, consistently saving huge areas from inundation during even the most extreme rain events.
When it isn't going to be active, the tank is cleared of silt and debris using earthmoving equipment lowered through the first shaft. It's an amazing piece of engineering and an impressive experience in person.
Each of the 59 pill-shaped columns weighs about 500 tonnes. In addition to their obvious function of supporting the roof and the 22 metres of ground above, they act as a counter weight against the buoyancy of the cistern in the surrounding groundwater.
It's difficult to appreciate in photos just how huge they are in real life, but to give a sense of scale each column is seven metres long and two metres wide, and an adult standing next to one would be about the same height as the buttressed base.
After an explanation of the system and its construction, we were invited to visit the underground surge tank.
Outside and at the far side of the field, an oblique concrete structure, like a sunken cube, serves as the entrance to the access shaft. A small black and gold plaque at the side listed the dimensions: 177.1 metres long, 78.0 metres wide, 25.4 metres high.
A hundred or so metal stairs down, the temperature steadily falling, then into the middle of the massive, dimly-lit cavern, where an array of giant columns towered, unshakeable.
Inside, greeted by staff, then up to the top floor where the control room is situated adjacent to a large display area, the floor of which features a satellite map detailing the path of the system's main tunnel.
The system is designed to prevent flooding of the basin during typhoons and heavy rain events by draining water from four rivers spread across the region. Each of the rivers has a 32 metre diameter, 60-70 metre deep underground shaft into which water can be diverted. These silos are joined 50 metres below the surface by a 6.4 kilometre long, 10 metre diameter tunnel that feeds the final shaft, which in turn connects to one end of the giant pressure control/surge tank. The tank acts as a buffer to regulate the flow of water so that it can be pumped into the Edo River 江戸川 at up to 200 cubic metres per second.
The taxi arrived, white gloves and auto-door, and we cruised across town, driver and interpreter chatting on the way, lamenting the loss of an ancient ginkgo 銀杏 tree in a recent storm.
There were no people to be seen or cars on the roads, but the tidiness of the farms that cover the countryside suggested a large workforce must exist.
At the facility, the large turbine and control building rises conspicuously at the end of an otherwise unremarkable soccer field. Hiding below it, at the end of kilometres of tunnels, is the mammoth “chika shinden” 地下神殿 (“underground temple”) – the surge tank that regulates the floodwater as it's pumped out of the channel.
At Minami-sakurai Station 南桜井駅 it was decided that a taxi was the easiest way to take the last few kilometres across town and through the farms to the riverside where the facility is located.
While waiting, I looked around a jutting granite monument nearby, rough hewn exterior, smooth inner planes with sakura 桜 pattern. Then, the nearby bicycle parking building, a common sight at stations in Japan, always exquisitely clean.
Taking the JR Saikyō Line 埼京線 about 40 minutes to Ōmiya Station 大宮駅, then changing to Tobu Railway 東武鉄道 and the Tobu Urban Park Line 東武アーバンパークライン (which had been renamed just the previous day from Tobu Noda Line 東武野田線), onward another 45 minutes to Minami-sakurai Station 南桜井駅, above.
During the trip we had plenty of time to get acquainted, chat about the history of the area and watch the concrete landscape skew and slide and eventually become greener and less dense as we reached the outer limits of Kasukabe 春日部市.