The vegetation thins as you ascend the Tongariro Saddle, switchbacking up solidified lava, until, finally, the only green things in sight are the Emerald Lakes - distant jewels so vivid against the stark backdrop that they appear to glow.
To your right is Mount Doom, where Frodo met his fate in Lord of the Rings (Mount Ngauruhoe to those not in the script); to the left are the Te Maari craters, which last erupted in 2012, damaging the roof of a hut a mile away. One hundred people were in the vicinity but miraculously no one was hurt.
Despite its 12-mile length and leg-burning climbs and descents, the Crossing is one of New Zealand's most popular hikes. Fit people can tackle it in five hours, though with time factored in for lunch and jaw-dropping stops, it usually takes at least six and maybe more. It is, without doubt, one of the most beautiful walks in the world, and the crowds that dot its winding route each day are evidence of that.
Tongariro National Park is a World Heritage Site and rightly so. Traversing the heights of the North Island, the park is the closest you can get to being on Mars while still on Earth. The early morning cloud seems to linger until you realize that the fog lifted hours ago, and that what you're looking at now is steam venting from the rock. Fumaroles gurgle sulphurous vapor into the air, yellow circles collecting at their bases like rings on a tree.
Strange colors abound: ochre and umber and buff and burgundy. Colors at home in the Martha Stewart paint aisle but eerily desolate on a volcanic plateau. The winds can be strong enough here to pick up pebbles - a natural emoliant to unprotected hikers. Every year someone dies of exposure, heart attack, or, in the case of one tourist in the '90s, scalding.
We spend lunch next to a lake fizzing like Ginger Ale and then begin the walk down the park's northern flank. The cumulus cloud layer below us is flat and uniform, as though viewed through the window of a 747. Soon we are in it, trudging in the mist as tussocks and brush reappear and the ground stops emiting heat.
The Crossing ends in temperate beech forest, and we feel, if only for a day, like time travelers, having traversed from this century into the Jurassic and back again, just in time for a beer before dinner.