Yellowstone is a high volcanic plateau in the northwest corner of Wyoming, the first national park in the world, and it holds about half of all the geysers on Earth, venting above a volcano that is still active under the ground. Old Faithful is the one everyone waits for. The visitor centre posts a predicted time with a twenty-minute margin, the semicircle of benches fills, and the geyser throws a column of boiling water thirty to fifty metres roughly every ninety minutes, as it has for as long as anyone has watched. Nearby, Grand Prismatic Spring is a lake of colour rings: the blue eye at the centre is water close to boiling in which nothing lives, dropping about fifty metres straight down, and the orange and yellow rings around it are mats of heat-loving microbes, each colour a different temperature. Morning Glory Pool was once deep blue all the way across; decades of coins and rubbish thrown in have cooled it, and its edges have gone yellow and orange. At Mammoth, boardwalks thread between stepped white terraces of limestone, a waterfall turned to stone.
The park is also the largest wildlife stage in the United States. In Lamar Valley the biggest wild bison herd in America crosses the road on its own schedule, and the rule is simple: you stop, switch off the engine and wait. Wolves were brought back in 1995 after seventy years' absence, and dawn in the same valley is when people with spotting scopes watch the packs; from September to mid-October bull elk bugle across the northern range to hold their harems. At Artist Point you stand over the Lower Falls of the Yellowstone River, twice the height of Niagara, at the head of a canyon whose walls are yellow, the colour that named the place. Under all of it is the caldera. The whole park is the roof of a supervolcano, and its floor rises and falls by centimetres from year to year as molten rock shifts below, which is why the ground steams, bubbles and smells of sulphur wherever you walk.