Tabayón del Mongayo Nature · Caso, Nalón, Asturias Image of Caso, not of the place itself. A fall of some sixty metres down which the Mongayo stream plunges in the south of the municipality, inside the Redes natural park; tabayón is precisely the Asturian word for waterfall, and it is a declared natural monument. Photo: Hernán Piñera (CC BY-SA 2.0) What to see and do in Caso