Got this info from the internet.
Most unusual of all is Playa de Gulpiyuri near Llanes. It has all the essentials of any beach anywhere: sand, salt-water, rocks, cliffs, waves and tide. But it is surrounded 360° by fields! This beach is over 100 metres from the sea.
Gulpiyuri is a deep hollow in the land linked to the sea by a natural underground channel. The tides drive the sea in and out providing all the essentials of the seaside, except a sea view.
The playa is only about 50 metres across but few people ever visit so you are unlikely to be bothered by crowds. There is next-to-no signposting (use Google Maps or GPS, coordinates 43.447622, -4.886127) and you will have to trek along a coastal footpath to get there. When you reach it, the sight of this amazing little place is your reward for taking the trouble to go where few people will follow. from http://www.littlehotels.co.uk/articles/articlegulpiyuri.php