Japan's 'underground temple' protects Tokyo from floods

In Kasukabe, a Japanese city some 30 kilometers (18 miles) from Tokyo, this gigantic chamber is part of an underground system of tunnels designed to protect the Japanese capital from flooding. Its dim interior and towering pillars evoke an ancient religious structure, inspiring names such as "the cathedral," "the shrine" or "the temple."

The construction of the complex, officially called the Metropolitan Area Outer Underground Discharge Channel, took 13 years to build and cost 230 billion yen (€1.4 billion/$1.63 billion). Since being commissioned in 2006, the system has already prevented over 150 billion yen's worth of flood damage, according to estimates by the Japanese Land Ministry.

The underground system protects the metropolis of Tokyo from flooding caused by heavy rainfall, typhoons and river overflows. It consists of gigantic water reservoirs and huge pumping stations that remove water quickly. According to Seita Emori, an environmental scientist at the University of Tokyo, rising temperatures will cause rainfall to increase in volume and intensity, and strain capacities.

The cave-like area has the capacity to hold the equivalent of almost 100 Olympic swimming pools. Inside are 59 massive pillars, each weighing 500 tons and measuring 18 meters (60 feet) high. When nearby rivers flood, the overflow travels through 6.3 kilometers (3.9 miles) of massive underground tunnels before collecting in a reservoir.

If the chamber isn't flooded, curious visitors can even take a look. Guided tours are available for tourists, and the location is also popular for films or photo shoots.

Visitors are amazed by the impressive size of the complex. Descending some six stories to the bottom of the chamber is a unique experience, with visitors entering a distinct microclimate. It's much cooler than the surface in summer, and warmer in winter. The huge columns are shrouded in clouds of mist.

But despite Tokyo's already sophisticated catchment system, the city could not prevent the flooding of more than 4,000 houses along the Nakagawa River in June 2023. These floods prompted the authorities to embark on a seven-year, 37.3 billion yen modernization project to strengthen dikes and water drainage systems.

In central Tokyo, a major project is also underway to link up canals to collect overflows from the Shirako and Kanda rivers starting in 2027, as part of Japan's climate preparedness plan. Tokyo's canal system is designed to handle rainfall of up to 75 millimeters (about 3 inches) per hour, but it's being increasingly overloaded by storms of up to 100 mm/h.