´The Wai-O-Tapu thermal area is part of a scenic reserve and has the largest area of surface thermal activity of any hydrothermal system in the Taupo volcanic zone. Covering some 18 sq.km, with the volcanic dome of Maungakakaramea (Rainbow Mountain) at its northern boundary, the area is literally covered with collapsed craters, cold and boiling pools of mud, water and steaming fumaroles. The general public sees only a very small portion. The area is associated with volcanic activity dating back about 160,000 years and is located right on the edge of the largest volcanic caldera (depression) within the active Taupo volcanic zone.
The Wai-O-Tapu stream forms an integral part of the drainage system which ultimately flows into the Waikato River and out to the Tasman Sea. Boiling springs and volcanic gases introduce numerous minerals into the water which account for no fish life in the stream.
Beneath the ground is a system of streams which are heated by magma left over from earlier eruptions. The water is so hot (temperatures of up to 300 degrees C have been recorded) that it absorbs minerals out of the rocks through which it passes and transports them to the surfaces steam where they are ultimately absorbed into the ground.
The so called ´rotten egg smell´of geothermal areas is associated with ´hydrogen sulphide´.