A Land of Fire and Ice
In the remote Mackenzie Mountains of Canada's Northwest Territories, a geological paradox exists. Here, geothermal hot springs with a constant temperature of 20°C emerge from deep within the Earth to meet a area defined by subarctic permafrost. This interaction creates the Nahanni Karst Towers, more accurately known as tufa mounds. The largest of these, the North Mound at Rabbitkettle Hotsprings, is the biggest in Canada, standing 30 meters high and 60 meters wide. It has been growing for an estimated 10,000 years, since the end of the last glaciation.
The process of their formation begins when geothermally heated water pushes up through faults in the limestone bedrock of the Nahanni Formation. Warmed by volcanic energy, this water dissolves high concentrations of calcium carbonate from the rock. When the mineral-rich water emerges at the surface and cools, the calcium carbonate precipitates out, hardening into the porous rock called tufa. Over millennia, this process has built up a series of multi-colored terraces and basins known as rimstone dams and gours, creating a area that appears unusual. This entire geological feature is made more unusual by the fact that it sits on a layer of permafrost.
Caves of Ice and Legend
Beneath the warm springs and growing tufa mounds lies another world: a network of ice caves. The Nahanni North Karst is considered one of the most important subarctic karst landscapes, with unique features found nowhere else. Caves like Grotte Valérie, which has tunnels spanning 1,900 meters, contain a bizarre combination of calcite formations and perennial ice structures. The interaction between the cold outside air, the permafrost, and the geothermal heat from below creates an environment for rare speleothems (cave formations) to grow. Inside, skeletons of Dall's sheep that became trapped hundreds or even thousands of years ago are preserved by the cold. One chamber, the "Gallery of Dead Sheep," contains the remains of around 100 animals.
This strange area has long been a place of deep cultural significance and mystery. To the local Dene people, the area is known as Gahnįhthah ("kettle"), a sacred place where offerings are left for good luck. Dene oral history also speaks of the Naha, a fierce tribe of mountain-dwellers who would raid lowland settlements before mysteriously vanishing. This, combined with the early 20th-century discovery of two headless gold prospectors, Frank and Willie McLeod, led to the region earning the name "The Valley of the Headless Men." These stories of giants and vanished peoples add a layer of human mystery to the already otherworldly geology.