The Zao mountain range, which forms the southern part of the border between Miyagi and Yamagata Prefectures, is a complex of volcanoes formed by repeated volcanic activity. The crater lake in the photo, called Okama, is located near the summit of the central part of the Zao mountain range, and active volcanic activity has been observed since the dawn of history. Incidentally, Okama is a deep hot pot in which rice is cooked, and the lake's shape resembles this, hence the name. This site has a caldera, a depression formed after an old volcano's big eruption. This lake was formed when rainwater and melted snow accumulated in the explosion crater that was formed in the caldera thereafter. Being 400 meters in diameter and 18 meters deep on average, this lake is void of aquatic life due to its highly acidic water. Very small eruptions from the bottom of the lake continue to this day, and the color of the lake water changes with each eruption. This color also changes with the weather and the seasons, which is why it is also called "Goshiki-numa" (five-colored swamp) by the locals.
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