Caviahue

Caviahue in Neuquén Province sits 2,000 metres up in the crater of Copahue volcano, surrounded by a landscape that smells of sulphur, steams from the ground, and contains a lake so acidic — pH 0.9, among the most acidic on Earth — that nothing lives in it except extremophile bacteria. The Copahue thermal spa uses the volcanic water directly, drawing it from underground before it cools, and the muds and sulphur pools have been treating joint and skin conditions since the nineteenth century. The ski resort on the crater's rim operates in winter above a valley that in summer becomes one of Patagonia's most unusual hiking circuits.

Caviahue sits at 1,900 metres inside the outer caldera of the Copahue volcano (2,997 metres), which remains active — its latest eruption cycle began in 2012 and has produced periodic ash emissions since. The Lago Caviahue, within the caldera, is a glacial lake with normal chemistry; the Lago Copahue, 8 kilometres south in the volcanic vent itself, is one of the world's most acidic crater lakes, with a pH of approximately 0.9 and a water temperature reaching 50°C. The Copahue thermal complex processes volcanic water at 70°C through dilution pools for therapeutic use, and the mud peloids extracted from the lake shore are exported to Argentine spa facilities nationwide. The Las Máquinas geothermal field, accessible on foot from the village, vents steam from ground fractures into a landscape of dead trees and ash-coloured soil.