Furnas is the name for sea caves which appears as a result of the erosion of a cliff in certain places which are eroded faster than the nearby rocks. We have a list of 22 furnas or sea caves in the three islands of Cíes archipelago.
They are caves made by the erosion of the sea using the parts where the granite is weaker. They are formed at an average height of the sea level where the erosive action of the swell is greater. They are called Furna da Porta, Buraco do Trapo, Cova do Inferno, Furna do Balcón, Cova dos Pesos, Furna de Marín, Furna das Lontras. They are often associated to legends.
The ledges of the top part of the furnas o sea caves are usually places where the European shag breeds.
