From the surface, it just looks like a park. A lawn and a soccer field. But if you take the vertical stairs down 50 meters underground, a completely different world appears. The sight of 59 18-meter-tall concrete pillars lined up in a space the size of two soccer fields—even in photos, I thought, "Is this really in Japan?" They say it's even more overwhelming in person.
Tokyo's low-lying areas used to suffer severe flood damage during typhoons and heavy rains. The Japanese government spent 13 years and 230 billion yen (approximately $1.5 billion USD) to build this very Metropolitan Area Outer Underground Discharge Channel. Instead of just holding water, it swallows overflowing river water whole and drains it into the Edo River. It truly deserves to be called the forefront of modern civil engineering.
Each Pillar Weighs 500 Tons
The structure of the Metropolitan Area Outer Underground Discharge Channel is nothing short of overwhelming in its scale. The core of the system, the Pressure-Adjusting Water Tank (Surge Tank), measures 177 meters long, 78 meters wide, and 18 meters high. This is larger than two soccer fields combined.
Supporting this space are 59 massive concrete pillars, each 18 meters tall and weighing 500 tons. A single pillar weighs as much as a large airplane. The sight of these pillars standing at regular intervals is reminiscent of the Parthenon in Greece, earning it the nickname 'Underground Temple.'
The entire facility stretches for 6.3 km, drawing water through underground tunnels to prevent surrounding small and medium-sized rivers from overflowing. What's most astonishing is its drainage capacity. At the heart of the facility are four 14,000-horsepower gas turbine pumps, modified from aircraft engines. These pumps can discharge 200 cubic meters of water per second into the Edo River—an incredible performance capable of emptying a 25-meter swimming pool in just one second.
How the System Activates During a Flood
The flood prevention system operates with precision, hidden from sight. When river water reaches dangerous levels, the system protects the city through the following three-step process.
Giant Vertical Shafts (Silos)
Overflowing river water flows into five giant vertical shafts, each 30 meters in diameter and 70 meters deep. A single shaft is large enough to contain the entire Statue of Liberty and serves as the primary buffer to dissipate the water's pressure.
The 6.3 km Connecting Tunnel
The underground tunnel connecting the five shafts runs 50 meters beneath the city. Water travels through this tunnel to the surge tank, and its flow rate is controlled during this process.
Surge Tank and Pump Drainage
Once stabilized in the surge tank, the water is safely discharged into a larger river by the giant pumps. The arrangement of the pillars, designed to withstand mechanical vibrations and water pressure, showcases the pinnacle of fluid dynamics.
Why Godzilla Was Filmed Here
Beyond its functional purpose, this facility has inspired countless artists with its unique engineering aesthetics. The immense concrete pillars, the cool air, and the geometric patterns created by light and shadow feel like a scene from a science fiction movie.
It has actually been used as a filming location for numerous movies, commercials, and music videos, as well as for tokusatsu (special effects) shows like Godzilla and Kamen Rider. Game developers have also borrowed its imagery to design dungeons for giant boss battles. The nickname 'Underground Temple' likely comes not just from its appearance, but from the sense of awe and sanctity one feels upon entering, similar to that experienced in a religious structure.
The Japanese government operates regular tour programs to share the value of this facility. It's considered one of the most unique attractions you can visit during a trip to Tokyo, and every year, tens of thousands of tourists and civil engineering experts from around the world visit Saitama to see this magnificent disaster prevention facility.
What to Know Before You Go
It's located in Kasukabe, Saitama Prefecture, which is over an hour by train from central Tokyo. From the east exit of Kasukabe Station, you can take a taxi or a bicycle. Be sure to check the official website in advance for the shuttle bus schedule.
Advance reservations on the official website are mandatory for entry. During typhoons or heavy rains, the facility is in actual operation, so tours are canceled. It's cool year-round 50 meters underground, so bring a light jacket even in summer. Sneakers are a must, as there are many stairs and the floor can be wet. Photography is allowed inside the surge tank, but the use of tripods may be restricted, so be sure to check.