“Onneto” means old lake or old pond. The lake, situated 623m above sea level, is part of Akan National Park and is one of the three great secret lakes in Hokkaido. The lake was formed when the river was dammed up due to the volcanic eruption of Mt. Meakan, the tallest active volcano in Akan Volcanic Complex. Lake Onneto is at the foot of Mt. Meakan. The minerals of the lake causes the change of color from emerald green to deep blue, forming a beautiful gem surrounded by a dense natural forest.