Wai-O-Tapu is a geothermal field south of Rotorua, on New Zealand's North Island, and every colour in it is a mineral coming out of cooling water. The centre of it is the Champagne Pool, a crater 65 metres across, blown out 900 years ago and now holding water at 74°C. Fine strings of carbon dioxide break the surface, which is where the name comes from. The boardwalk carries you to the edge, close enough that the warmth reaches your face. From the wider viewpoint the pool sorts itself into zones: grey-green centre, orange shelf around the shallows, bleached apron beyond. Those bands are a record of temperature. Where the water cools at the rim it drops its arsenic and antimony sulphides, and that precipitate is the orange.
The overflow does the rest of the painting. It spreads across the Artist's Palette, staining the ground wherever another mineral comes out, then runs down Primrose Terrace, the country's largest silica terrace. A film of hot water is still adding to that rock under your feet. Further on the track takes you past mud pots thudding and craters crusted sulphur-yellow. At the far turn is the Devil's Bath, a flooded crater fed by drainage from the pool above. Its water is an opaque yellow-green, and sulphur and iron shift it between lime and near-fluorescent, depending on how much of each it carries that day. Lady Knox, three kilometres away, does not go off on its own. At a quarter past ten a warden tips surfactant into the vent, breaking the tension between the cool water on top and the hot water below. Within a minute the cone foams over, and the jet can stand twenty metres up for an hour. Prisoners washing clothes in the spring found that out. The orange shelf is doing something rarer still. As that water cools the metals ride down with the sulphides: sinter assayed at up to 540 grams of gold per tonne, an ore deposit forming a few metres from where you stand.