Wai-O-Tapu's Champagne Pool is an explosion crater that hot water blew open roughly 900 years ago: 65 metres across, 62 metres deep, held at a steady 74°C. Carbon dioxide rises through it in fine bubbles — the reason for the name — and where the water cools at the shallow rim it drops a shelf of arsenic and antimony sulfides, orange so saturated it looks painted on. That crust is literally precious. The sinter beside the pool has been measured at up to 540 grams of gold per tonne, and geologists come here to watch an ore deposit forming in the open. The pool is one vent in an 18-square-kilometre field 27 kilometres south of Rotorua, where the crust of the Taupō Volcanic Zone runs thin enough for water, gas and mud to keep breaking through. Māori named the place Wai-O-Tapu — sacred waters.
A thin sheet of the pool's overflow does the rest of the work. It spreads across the Artist's Palette, a pale flat streaked orange and green wherever a mineral drops out of the cooling water, then slides down Primrose Terrace, the largest silica terrace in the country. What is left collects in the Devil's Bath, a pond of unreal yellow-green that shifts shade with the sun and with how much sulphur and iron the water carries. Three boardwalk loops, 45 to 90 minutes, run past craters bleached yellow, mud thudding in slow motion, and ground that steams wherever you look. The one staged act is Lady Knox Geyser: a warden drops surfactant into its vent at 10:15 each morning and the column climbs up to 20 metres — a trick prison labourers stumbled on in 1901 while washing clothes in the hot spring. The reserve is now run by Te Arawa Group Holdings, the local iwi's company.