Rain that falls today
surfaces again decades later.

Mount Fuji is built from layer upon layer of highly permeable basaltic lava and sandy, gravelly scoria. Rain and snow soak in as if absorbed by a sponge, becoming groundwater with almost no surface runoff. As a result, Fuji has very few rivers, but an enormous amount of spring water emerges at points all around its base — a distinctive water cycle.

Water that seeps underground travels slowly along impermeable layers between lava beds, taking decades to filter through before emerging as spring water at the foot of the mountain. The water gushing out today may include water that traveled through the younger New Fuji lava layer (roughly 10,000 years old) mixed with water that passed through the much older Old Fuji strata (roughly 100,000 years old).

The eight spring-fed ponds scattered across Oshino village in Yamanashi Prefecture, collectively known as "Oshino Hakkai," formed where Fuji's underground water surfaces, and are themselves registered as a component asset of Mount Fuji's World Heritage listing. The area once held a lake called "Lake Utsu," which shrank and dried up over time due to eruptions and land shifts, leaving only these spring points as ponds.

Each of the eight ponds has its own name — "Deguchi-ike," "Okama-ike," "Sokonashi-ike," and others — and they also carry a history as a pilgrimage site, where Fuji-ko devotees would visit all eight as part of their sacred circuit. Water temperature stays around 13°C year-round, and the crystal-clear water with its bluish tint draws many photography enthusiasts.

Numerous major beverage and food manufacturing plants are located throughout the foothills of Mount Fuji, in both Shizuoka and Yamanashi Prefectures. This is no accident. Fuji's groundwater maintains stable temperature and quality year-round, and allows for reliable extraction of large volumes of high-quality groundwater — extremely favorable conditions for beverage production. For industries where water quality directly shapes product taste — mineral water, beer, whisky — the Fuji foothills are considered among Japan's finest locations.

This groundwater has also blessed agriculture and aquaculture. Spring water that stays consistently cool is well-suited to growing wasabi and farming cold-water fish like rainbow trout, and both Yamanashi and Shizuoka have long made active use of it.

Beneath Mount Fuji lies a network of caves carved by flowing lava — also part of the path water takes underground. The next page explores this hidden world.