Even in retreat, the scale is staggering. The Tasman Glacier still stretches 23.5 kilometers through Aoraki / Mount Cook National Park, up to 4 kilometers wide and 600 meters thick in places. It begins at 3,000 meters above sea level, where winter snowfall can pile up to 50 meters deep. After the summer melt, about 7 meters of compacted snow remain at the glacier's head, slowly compressed into the blue ice that feeds the river below. Five tributary glaciers -- Forrest Ross, Kaufmann, Haast, Hochstetter, and Ball -- once added their own mass to the main flow. The meltwater drains into Tasman Lake, joins the braided Tasman River, and eventually reaches Lake Pukaki, the turquoise reservoir whose color comes from the fine rock flour ground by this very glacier.

For the entire 20th century, the Tasman held its ground. Then, in the 1990s, something shifted. The terminus began pulling back at roughly 180 meters per year. That pace has since accelerated dramatically, with recent calculations putting the retreat rate between 477 meters per year and higher. Scientists estimate the glacier will eventually disappear entirely, with Tasman Lake reaching its maximum extent within 10 to 19 years. The retreat is not always gradual. On 22 February 2011, the same earthquake that devastated Christchurch triggered a massive calving event here: between 30 and 40 million metric tons of ice broke from the terminal face and crashed into the lake, generating tsunami waves up to 3.5 meters that struck boats on the water.

Tourism and the Tasman have a long partnership. Heli-skiing tours have operated on the upper glacier since the early 1970s, ferrying adventurers to runs that few valleys on Earth can match. The tributary Ball Glacier was once popular enough to host national skiing championships in the 1930s, but it has since retreated too far to be safely reached. The glacier's losses are reshaping what visitors come to see. The Ball Shelter Track leads along the western flank, separated from Tasman Lake by a towering moraine wall. About 6 kilometers in, the track climbs high enough for the view to open up -- and there, between the moraine walls, lies the rock-covered lower glacier, still 2.5 kilometers wide, its surface cloaked in gray debris that belies the ancient ice beneath.

What makes the Tasman Glacier compelling is not just its size but its velocity of change. The moraine walls that once contained a thick, stable tongue of ice now tower higher than they should, their increased height a visual record of how much the glacier has thinned. The Murchison Glacier approaches from the northeast and flows alongside the Tasman outside the moraine wall, their meltwaters merging before running south. Where the two glaciers once pressed against each other, there is now open water and exposed gravel. The Tasman is becoming something else -- a lake, a river system, a canyon of moraine -- and it is doing so on a timeline measured in years, not centuries. For anyone who flies over this valley, the transformation is visible in a single glance: ice giving way to water, permanence dissolving into flow.