The Rainiest Place On Earth

A giant rainfall simulator in Tsukuba, Japan can reproduce the most intense rainfall ever recorded—down to the millimeter per hour—so researchers can stress-test flood and landslide risk under controlled, repeatable conditions. The facility matters because Japan’s typhoons can dump enormous volumes of rain in short windows, overwhelming drainage systems and destabilizing slopes, with consequences measured in evacuations, deaths, and massive economic losses.

Japan’s wet extremes are not just a curiosity: Yakushima Island can receive up to 10 meters of rain annually, while the Amazon averages about three meters. Most of the time, rainfall soaks in or evaporates without incident. But after typhoons, water arrives faster than soils and infrastructure can absorb it. In July 2018, Typhoon Prapiroon triggered floods across Japan; some regions saw nearly two meters of rain in 10 days, forcing more than 8 million people to evacuate and contributing to over 200 deaths and more than 1 trillion yen in damage. Tokyo faces a particular vulnerability because it sits amid more than a hundred rivers; underground pipes and tunnels feed an enormous storage tank. During Typhoon Hagibis in October 2019, the system diverted 12 million cubic meters of water after the city received over 200 millimeters in under 48 hours, preventing an estimated $1.7 billion in damage.

The simulator’s core capability is precision. A warehouse-like structure houses 550 nozzles that can dial rainfall intensity from 15 millimeters per hour up to 300 millimeters per hour—matching the worst one-hour rainfall record set in Holt, Missouri in 1947 (305 millimeters in an hour). Researchers can also vary raindrop size using nozzles with four hole diameters, since drop size affects fall speed and how water impacts surfaces. The physics matters: gravity and air resistance set terminal velocity, and large drops fall faster, while very large drops deform, flatten, and break apart.

Landslides add another layer of complexity. NIED researchers have identified over 700,000 locations where landslides threaten homes, but predicting when a slope will fail—slow slide versus rapid flow—depends on interacting factors: slope geometry, soil type and minerals, vegetation, and how water pressure builds within pores. The facility can even move on railway tracks and tilt into set angles (such as 20 and 30 degrees) to run shallow and steeper slope experiments with controlled soil layers. A key correction to common intuition is that water is not an “anti-friction” lubricant; instead, rainfall fills soil pores, increases pore water pressure, and reduces friction between grains, allowing gravity to overcome resistance.

Beyond geohazards, the same repeatable rain environment supports engineering tests for drones and self-driving cars. Rain can reduce camera and LIDAR performance by lowering visibility and sensor accuracy, so controlled storms help developers improve detection hardware and software. With climate change driving more frequent and intense downpours—Japan’s 50 millimeters per hour rain events becoming 40% more common and double-intensity events up 70%—the simulator’s role is likely to expand. The work aims at prevention and better design, while also underscoring that long-term risk reduction depends on addressing the root causes of climate change, not only its impacts.