Beach of Arnao Nature · Castrillón, Avilés, Asturias Beneath the headframe of the old mine lies one of Europe's richest Devonian fossil sites, with remains around 415 million years old visible on the rocky rasa itself when the tide goes out; this is the Arnao platform, a set of fossils from coralligenous formations that once formed a reef, with three distinct types of rock layers. The beach, some 300-350 metres long and little built up, lies in the town of Arnao, in the Asturian concejo of Castrillón, and preserves the memory of the former Real Compañía Asturiana de Minas. With fine, whitish sand and a swell ranging from moderate to strong that attracts surfers, it has a 500-metre seafront promenade and a natural pool known as La Hollina. Photo: LMLM (CC BY-SA 3.0) What to see and do in Castrillón