The lake water is highly acidic due to the volcanic gases escaping from the lake bed.
The 'Five-Color' phenomenon is caused by the refraction of sunlight through suspended volcanic minerals.
Mount Zao is classified as an active volcano, and the lake is monitored by the Japan Meteorological Agency for seismic activity.
The crater rim offers a vantage point where visitors can see both the Miyagi and Yamagata sides of the mountain range simultaneously.
Heavy snowfall often buries the access roads and the crater observation deck, requiring the area to close for several months each year.
Okama is a crater lake situated in the summit caldera of Mount Zao, an active volcano on the border of Miyagi and Yamagata prefectures. The lake is famously known as the 'Five-Color Pond' because its water color shifts between shades of emerald green and turquoise depending on the intensity of sunlight and mineral content. It measures approximately 360 meters in diameter and 60 meters in depth. The lake is highly acidic with a pH of about 3.5, which prevents any aquatic life from inhabiting its waters. It sits at an elevation of approximately 1,600 meters above sea level within the Zao Quasi-National Park. The surrounding landscape features jagged, volcanic rock formations typical of a stratovolcano's crater rim. Access is provided via the Zao Echo Line and Zao Highline, which connect to the parking area near the summit. The water temperature remains consistently low year-round, often near freezing.
The elevated observation deck overlooking the lake at the edge of the crater rim.
Bring a windproof jacket, as temperatures at the summit are significantly lower than in the valley, even in mid-summer.
Check the local volcanic activity level and road status on the official Zao Town tourism website before departing.
Sun protection is essential, as there is no shade on the crater rim and UV exposure is high at this altitude.
Do not attempt to hike to the lake's edge, as the walls are unstable and entry into the crater is prohibited.
Access roads are closed during the winter season due to heavy snowfall; typically reopens in late spring.
Stay within the designated roped-off observation areas for safety due to the steep crater walls and unpredictable volcanic gas.
