Mysterious Hole In Hokkaido Mountains Revealed

Nov 10, 2025 | News On Japan

HOKKAIDO, Nov 10, 2025 (News On Japan) - Aerial images of Hokkaido showed a solitary round “hole” in the mountains near Mt. Yotei, leading a reporting team to Kyogoku Town where an unpaved forest road ended at a dam and, beyond a locked gate and warning signs, at a restricted facility on a ridge that turned out to be a perfectly circular reservoir built on the mountaintop.

Staff explained the site is the upper regulating reservoir for a pumped-storage hydroelectric system. Water stored at the summit is routed through conduits to turbines far below to generate power, then collected in the lower dam; when demand is low, the same machines are driven in reverse to pump the water back uphill so it can be used again. The reservoir’s perimeter is about 1,500 meters, with a storage volume on the order of 4.4 million cubic meters—roughly three and a half Tokyo Domes—creating a vast buffer of potential energy.

The heart of the system lies some 400 meters underground inside a cavernous powerhouse reached through a steep access tunnel. There, water from the upper pool spins the turbines at roughly 500 revolutions per minute to produce electricity. The machinery can switch modes, functioning as generators when water flows downward and as pumps when power is abundant.

Hokkaido Electric Power monitors the network around the clock from a control center in Sapporo, adjusting output moment by moment to keep supply and demand in balance. With solar output prone to rapid swings when clouds pass, the pumped-storage plant absorbs surplus electricity by lifting water uphill and then releases it during peaks, acting, in effect, like a giant rechargeable battery that converts electricity into stored water energy and back again.

The restricted access and sirens heard on approach are part of routine operations—a measure also used to deter bears in the otherwise quiet mountain area. What looked like a simple hole from the sky proved to be the top of an intricate energy storage system designed to stabilize Hokkaido’s power supply.

Source: 北海道ニュースUHB

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