Lago Fagnano is a massive glacial lake straddling the border of Argentina and Chile on the main island of Tierra del Fuego. The lake stretches approximately 98 kilometers east to west, with a total surface area of 645 km², though only the westernmost 13.5 kilometers (roughly 39 km²) fall within Chilean territory in the Magallanes y la Antártica Chilena Region. Despite the relatively small Chilean portion, the lake's scale is immediately apparent on the water: it reaches a maximum depth of 449 meters and an average depth of 193.8 meters, giving it the character of a freshwater sea rather than a typical inland lake. Strong waves and whitecaps develop readily, particularly when the persistent Tierra del Fuego winds pick up.

The southern bank is steep, while the northern shore expands into a wide, flat piedmont with views of surrounding plateaus. The landscape shifts dramatically with the seasons — snow and frost dominate in winter, while warmer months bring a range of colors from blue and green to red and yellow in the surrounding vegetation. Wildlife includes albatross fishing on the lake surface and introduced beavers that have significantly altered the surrounding forest with their dams and felled trees. The calafate berry bush is a notable feature of the shoreline flora.

Chilean access to the lake runs through the central-southern part of the Tierra del Fuego archipelago via roads passing through forests, lakes, and mountains. The western end of the lake is the outlet of the Azopardo River, which drains toward the Almirantazgo Fjord (also referred to locally as Caleta María, approximately 12 kilometers from the lake's western shore). Road Y-85 provides access to the Chilean lakeshore.

The months of November through April offer the most daylight, ranging from roughly 12 to 17 hours, making this the most practical window for paddling. June through August brings only around 7 hours of daylight. Wind is a defining feature of the weather year-round and should be treated as a primary hazard; conditions can change quickly and generate significant wave action on this large, exposed body of water. Paddlers should plan accordingly for cold water temperatures given the lake's depth and sub-Antarctic latitude.