According to a Selk'nammyth the lake was created alongside the Strait of Magellan and Beagle Channel in places where slingshots fell on earth during Taiyín's fight with a witch who was said to have "retained the waters and the foods".
Fagnano Lake, from north to south, in its Chilean part
References
Wikimedia Commons has media related to Lake Fagnano.
^Montecino Aguirre, Sonia (2015). "Canal de Beagle". Mitos de Chile: Enciclopedia de seres, apariciones y encantos (in Spanish). Catalonia. p. 125. ISBN978-956-324-375-8.
Niemeyer, Hans; Cereceda, Pilar (1983). Geografía de Chile — Tomo VIII: Hidrografía (1º edición, Santiago de Chile: Instituto Geográfico Militar ed.).