Mount Ruapehu Crater Lake
Mt Ruapehu is the largest active volcano in New Zealand and is located at the southern end of the Taupo Volcanic Zone. , Ruapehu is the highest peak in the North Island (2797m), with several subsidiary peaks and several small glaciers. Three summit craters have been volcanically active in the last 10,000 years. The active vent is now beneath the crater lake of South Crater. The crater lake contains warm, acidic water that is fed by snow melt.
....read on at GNS science web site