Mount Zaō(蔵王山,Zaō-san?) is a complex volcano on the border between Yamagata Prefecture and Miyagi Prefecture in Japan. It consists of a cluster of stratovolcanoes and is the most active volcano in northern Honshū. 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 metres (1,200 ft) in diameter and 60 m (200 ft) deep, and is one of the main tourist attractions in the area.
One striking feature of Zaō's famous ski resorts is the snow monsters(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 mountains in Japan.
Rotaria rotatoria and Pinnularia spp. are found in the acidic Okama Lake.