(An underground water infrastructure project in Kasukabe, Saitama, Japan)
Hidden beneath the suburban landscape of Kasukabe, Saitama Prefecture, lies one of Japan’s most impressive engineering feats—the “Underground Temple.” Its official, less magical name, the Metropolitan Area Outer Underground Discharge Channel (or G-Cans Project), was built to safeguard Tokyo and its surrounding areas from devastating floods.
The colossal structure is both a technological marvel and an unlikely tourist destination thanks to its scale and almost mythic design. It’s a must-see for anyone intrigued by architecture, urban planning or the sheer ingenuity behind keeping a city safe.
At first glance, it might sound more like a science fiction set than a real location. But the G-Cans Project is a flood mitigation system on a scale that few other countries could rival. Completed in 2006 after nearly 14 years of construction and ¥230 billion invested, the system’s purpose is to prevent flooding in Tokyo’s low-lying areas, particularly during typhoons and heavy rainfall.
How Japan Built the World’s Largest Flood Control System
At its core is a 177-meter-long pressure-adjusting water tank, dubbed the “Underground Temple” for its resemblance to a grand cathedral. Its towering 18-meter-high pillars, each weighing an astonishing 500 tons, and cavernous spaces create an almost spiritual atmosphere. The size and symmetry make it feel more like a sacred space than an industrial one. Though its purpose is entirely practical.
Though the facility’s primary purpose is flood control, its striking design has turned it into an unconventional yet highly sought-after tourist attraction. You must book a guided tour in advance through the official website. These tours are conducted on weekdays only, primarily in Japanese. Though the visuals alone make it a worthwhile experience, even if you don’t understand the language.
What to Expect on the G-Cans Tour
The tour begins with a briefing on the project’s history and mechanics. Then visitors descend into the depths of the pressure-adjusting water tank. Standing among the 59 massive columns, with dim light filtering through, you can’t help but feel like you’re in a post-apocalyptic film set—or perhaps the lair of a movie supervillain.
The system itself doesn’t just stop at the main tank. It features 6.4 kilometers of tunnels and five enormous vertical shafts—roughly 70 meters deep and 30 meters in diameter. To put the scale in perspective: these shafts are large enough to fit the Statue of Liberty lying on its side. When standing at the bottom of one, it’s hard not to feel dwarfed by the enormity of the design.
When heavy rain threatens Tokyo, water from overflowing rivers is diverted into the system’s vertical shafts, where gravity does most of the work. Once inside, the floodwaters travel through the network of tunnels to the pressure tank, which helps balance the flow. Pumps—each the size of a small house—then expel the water into the Edo River, protecting urban areas from inundation.
This carefully orchestrated process can move up to 200 tons of water per second. That’s like draining an Olympic-sized swimming pool in just 12 seconds. Managing such immense volumes of water requires precise coordination, and during peak flood events, a team of around 30 operators works together to ensure everything runs smoothly.
The pressure tank’s towering columns are also designed to withstand incredible forces during floods, a testament to the engineers’ foresight.
Planning Your Visit to Tokyo’s Underground Temple
Location: Near Minami-Sakurai Station on the Tobu Urban Park Line, about 60–75 minutes from central Tokyo.
Address: 720 Kamikanasaki, Kasukabe, Saitama
Reservations: Required in advance through the official website.
Cost: Approximately ¥3,000 per person.
Tour Availability: Weekdays only, typically between 10 am and 4 pm. Of course, it’s worth noting that the facility occasionally closes during periods of heavy rainfall. Its primary purpose as a flood control system takes precedence.
Website: gaikaku.jp
