The so-called furnas are sea caves, which are the result of erosion on the cliff at points corroded faster than the rocks nearby.
We have classified classified over 30 furnas between Onza and Ons islands.
Caves and furnas are produced by sea erosion, using weak granite areas and they are formed at an average height at sea level, where the erosive action of the waves is greater. They have names like furna of Estripeiral, Xubenco, Centulo, Cova do Lobo, Castelo, Coviñas, Fedorentos or Buraco do Inferno often associated with legends.
The shelves of the top part of the furnas or sea caves tend to be breeding grounds of European shag.
