Yellowstone was the world's first national park, set aside by the United States in 1872, and the first thing to understand is that the ground here is alive. The park sits on a huge volcanic caldera left by eruptions that blew out an enormous chunk of the Rockies, and the heat still rising from below drives roughly half of the world's geysers, along with hot springs, mud pots and steaming vents. Old Faithful, in the south-west of the park, is the famous one, going off at fairly regular intervals through the day.
Riding here is a slow business, and gladly so. The Grand Loop road is a figure of eight, and speed limits are low, often around forty-five miles an hour, with good reason: bison walk down the middle of the tarmac, elk graze on the verge, and traffic stops dead whenever somebody spots a bear. Surfaces are decent, though frost heaves and patched sections appear, and the road is much busier in July and August than it looks on a map. Go early, around dawn, and you will often have the best light, the thinnest crowds and the most wildlife. Weather turns fast at over two thousand metres. Snow can fall in any month, and most roads close to cars from late autumn until spring, so check before you set out.
The landscape moves from lodgepole pine forest to open valleys such as Hayden and Lamar, where bison and wolves are the main attraction, to the deep golden canyon of the Yellowstone River with its two waterfalls. The sulphurous smell around the geyser basins is unforgettable, and the colours of Grand Prismatic Spring, which come from heat-loving bacteria, look almost unreal on a bright day. Keep to the boardwalks. Water in these pools can be scalding and acidic, and people have died by stepping off the path.
For fuel, the nearest town outside the park is West Yellowstone, about thirty kilometres from here, and there are a few service stations inside at the major villages, though they cost more and can have queues. Do not rely on them if you are low. Carry water, a warm layer and a rain jacket whatever the forecast, and respect the distance rule for wildlife, which is a good deal longer than most people think.