Qualla: Mount Kusatsu-Shirane

Mount Kusatsu-Shirane: Introduction

Mon, 16 Feb 2026 00:00:00 GMT

Photo credit No machine-readable author provided. Takkitakitaki assumed (based on copyright claims)., Public domain. The water should not be that color. At 2,165 meters above sea level, cradled inside a volcanic crater in Gunma Prefecture, Yu-gama glows an impossible shade of emerald green. Rafts of bright yellow sulfur drift across its surface like strange lily pads. The lake measures 270 meters wide and plunges 27 meters deep, and its pH hovers around 1.2, making it one of the most acidic natural bodies of water anywhere on the planet. Mount Kusatsu-Shirane earned its name to distinguish it from Mount Nikko-Shirane on the other side of Gunma Prefecture, but there is nothing ordinary about this volcano. Positioned just north of the powerful Mount Asama, its summit consists of overlapping pyroclastic cones and three crater lakes, a landscape that looks less like the Japanese highlands and more like another world entirely.

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Image: No machine-readable author provided. Takkitakitaki assumed (based on copyright claims). | Public domain

Mount Kusatsu-Shirane: A Lake That Burns

Mon, 16 Feb 2026 00:00:00 GMT

Photo credit MaedaAkihiko, CC BY-SA 4.0. Yu-gama is the largest and most striking of Kusatsu-Shirane's three crater lakes. Its emerald color comes from dissolved minerals and colloidal sulfur suspended in water so acidic it would dissolve metal. Hydrothermal vents on the lake bottom heat the water from below. Between 19...

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Image: MaedaAkihiko | CC BY-SA 4.0

Mount Kusatsu-Shirane: The Mountain That Surprised Everyone

Mon, 16 Feb 2026 00:00:00 GMT

Photo credit Yuhei Mukoyama, CC BY-SA 3.0. On the morning of January 23, 2018, the volcano did something no one expected. The eruption came not from the heavily monitored Shirane summit and its famous crater lakes, but from Moto-Shirane, a separate pyroclastic cone group on the volcano's southern flank that had shown no s...

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Image: Yuhei Mukoyama | CC BY-SA 3.0

Mount Kusatsu-Shirane: Living Alongside Fire

Mon, 16 Feb 2026 00:00:00 GMT

Photo credit Batholith, Public domain. Since 1805, Mount Kusatsu-Shirane has erupted on average every 10.5 years, a rhythm that the people of Kusatsu town have learned to live with. The town itself sits at roughly 1,200 meters elevation, one of Japan's most celebrated onsen resort towns, where the same geothermal forc...

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Image: Batholith | Public domain

Mount Kusatsu-Shirane: The View from Above

Mon, 16 Feb 2026 00:00:00 GMT

Photo credit Alexrk2, CC BY-SA 3.0. From the air, the volcano's structure reveals itself in a way the ground never allows. The three pyroclastic cone groups, Shirane, Ainomine, and Moto-Shirane, line up from north to south like vertebrae along a spine. Yu-gama's unnatural green stands out against the gray-brown vol...

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Image: Alexrk2 | CC BY-SA 3.0