Yugama, at the peak of Kusatsu-Shirane volcano(2,160m) in Gunma Prefecture, is the most acidic lake in the world

Yugama at the peak of the Kusatsu-Shirane volcano (2,160m) in Gunma Prefecture is the most acidic, about pH1, volcanic lake. This lake measures 300m in diameter, 30m in depth and is almost circular in shape. The lake contains stagnant water in the crater, spews hydrogen sulfide and water vapor, and is accumulating sulfur on the bottom.

Mount Shirane is a part of Joshin’etsu Kogen National Park. Yugama’s surface is an impressive emerald green, so it is popular amongst tourists. The emerald green color because of the influence of the iron ion melting in the lake water and of sulfur and many minerals floating in it. However, if one actually samples water from the lake and holds it to the light, you can’t see its color.

Mount Shirane is an active volcano in the Nasu Volcanic Zone. Eruption records, first recorded in 1805(the year of Bunka 2), indicate that Yugama erupted quite often, including in 1805.

In addition to Yugama, there are Mizugama and Karagama at the peak. In the year of Showa 7, two sulfur miners were killed by eruption rock debris. Seven were injured and volcanic mud also flowed. All over Mount Shirane was famous for the production of sulfur but now the mining has closed. In this area there are many famous hot-spring resorts, Kusatsu, Manza and so on.