Yellowstone sits on the roof of a supervolcano, and the roof moves. The floor of this plateau in northwest Wyoming lifts and settles by centimetres from year to year as molten rock shifts underneath, and the steaming basins are where the system lets off pressure. About half the geysers on Earth are here. Old Faithful is the one with a timetable. The visitor centre posts a predicted window with twenty minutes either way, so you take a bench long before it. The vent hisses and spits false starts, then the column climbs forty metres and holds. Grand Prismatic reads as a set of rings. The blue eye at the middle is water too close to boiling for anything to live in, dropping fifty metres straight down, and the orange and red bands around it are mats of microbes, one colour per temperature.
The animals work at the same scale. In Lamar Valley the largest wild bison herd in the country crosses the road whenever it likes, and the arrangement is that you stop, switch off and wait. From late July the bulls bellow through the rut, a rumble that carries right across the grass. At Mammoth, hot water rising through limestone drops its minerals as it cools, building white terraces like a waterfall turned to stone. They still shift enough to change shape from year to year. At Artist Point the river falls ninety-four metres between cliffs stained yellow and rust by iron, which is where it and the park got their name. Congress set all this aside in 1872, when no country had yet protected land purely as a public park. Fourteen wolves were put back in 1995, seventy years after the last were shot out. Learning what leaving a place alone means has taken longer. Morning Glory Pool was blue right across within living memory, until decades of coins and rubbish thrown into its vent cooled the water and orange microbes crept in from the rim toward the centre. What you photograph there now is a record of what earlier visitors did to it.