From December to February, the buses from Coyhaique disgorge travelers onto the main road, where there is no station, only a tourist office and a row of agencies advertising boat tours. Many of the visitors are Chileans on their own pilgrimage south, and the little grid of guesthouses fills fast enough that booking ahead becomes wise. Then the season ends. The crowds thin, the lakefront empties, and shutters go up across half the businesses. Winter here is long and quiet, the village returned to the few who stay year-round. Puerto Río Tranquilo lives a double life, and both versions are true: the sleepy outpost on the Carretera Austral and the brief, bright tourist boomtown that materializes when the southern sun finally lingers.

The reason almost everyone comes is a short boat ride away. The Marble Caves, the Capilla and Catedral de Mármol, rise straight from the lake as smooth, swirling rock, their undersides glowing turquoise from the water reflecting up into them. They are real marble, roughly 94 percent calcium carbonate, and the lake has been dissolving and polishing them for around 6,000 years. Wave action widens every crack; the water carries away what it loosens. In the small outboard boats that hold about ten people, guides nose right inside the hollows, where the walls run blue and gray and pink. The larger, more comfortable excursion boats stay outside. Cheaper canoe tours leave from Puerto Mármol, six kilometers south. However you reach them, the effect is the same disorienting beauty: stone that looks liquid, floating on water that looks like sky.

The lake is only one of the town's two great draws. West of here, the Northern Patagonian Ice Field spills toward the Pacific, and from Río Tranquilo a rough road runs up the Exploradores Valley to the boats that carry visitors out to the San Rafael Glacier. It is the tidewater glacier nearest the equator, one of the most actively calving glaciers on Earth, dropping blocks of ancient ice into a lagoon where they drift as bergs. Operators in town run trips of one, two, and three days to reach it. The journey is long and the weather unreliable, but the payoff is a wall of blue ice grinding into the sea at the edge of one of the last great ice masses outside the poles.

This is the deep Carretera Austral, and the town reminds you of it constantly. There is one ATM, tucked inside the petrol station kiosk, and it charges to use it; smart travelers carry cash from Coyhaique, Cochrane, or Chile Chico. The Wi-Fi in the hostels strains under the summer load. Kayaks rent by the half day, hostels cook dinner if you ask in advance, and the supermarket keeps the split hours common across rural Chile, closing through the long lunch and reopening in the evening. None of it is polished. That roughness is part of the appeal: Puerto Río Tranquilo is a working frontier village that happens to sit beside two of Patagonia's most extraordinary sights.