Tongariro is a national park in the middle of New Zealand's North Island built on three volcanoes, and the way to see it is to walk across one of them in a day. The Tongariro Alpine Crossing runs 19.4 kilometres from tussock flats up through old lava flows. Halfway along it crosses South Crater, a flat pan of grey ash with steam drifting from vents at the edges and not a plant in sight. The climb tops out on the rim of the Red Crater, and the way down is a steep slope of loose scoria that slides under your boots. At the bottom are the Emerald Lakes, and the water at your boots glows an unreal green. Nothing living makes that colour: acid groundwater strips iron and sulphur out of the rock, and a pale bed underneath throws the light back up. Sulphur hangs in the air, and the ground around the shore is warm enough for you to sit on. Further along lies the Blue Lake, which is sacred: nobody eats beside it or touches the water.
Ngauruhoe stands beside the track all morning, a near-perfect grey cone, clearest from the lava fields before cloud wraps it. Most of the world knows it as Mount Doom. It looks like a mountain of its own and is not one, just Tongariro's youngest vent, built over the last 2,500 years. The park exists at all because these mountains are sacred to Ngāti Tūwharetoa. In 1887 their paramount chief, Te Heuheu Tūkino IV, handed the three peaks to the nation rather than watch them go through the Native Land Court and come out cut into blocks and sold. Ruapehu, the biggest of the three, carries New Zealand's largest ski areas. It is also live. After a disaster in 1953 its crater lake was wired with an alarm. When the dam broke again in 2007 the sirens closed the roads and railway in time, and nobody was hurt. So in winter you ride a chairlift over lava bluffs and small glaciers, with that same crater lake steaming above the top of the runs.