Mount Zaō

The Zaō Mountains (蔵王連峰), commonly called Mount Zaō, are a complex cluster of stratovolcanoes on the border between Yamagata Prefecture and Miyagi Prefecture in Japan. The central volcano of the group includes several lava domes and a tuff cone, Goshiki-dake, which contains a crater lake named "Okama". Also known as the "Five Color Pond" (五色沼, goshiki numa) because it changes color depending on the weather, it lies in a crater formed by a volcanic eruption in the 1720s. The lake is 360 m in diameter and 60 m deep, and is one of the main tourist attractions in the area.

One striking feature of Zaō's famous ski resorts are the "frost-covered trees" (樹氷, juhyō) that appear in mid-winter. Strong wind over the nearby lake fling water droplets which freeze against the trees and their branches, until near-horizontal icicles begin to form. Falling snow settles on the ice formations, and the end result is a grotesque figure of a tree. The effect of a full forest of such trees gives visitors a ghostly impression. Zaō is one of the 100 Famous Japanese Mountains.

Rotaria rotatoria and Pinnularia spp.  are found in the acidic Okama Lake.