Grotta di Nettuno (Italy). 'Whether you glide in by boat from Alghero or take the vertiginous 656-step staircase that zigzags down 110m of sheer cliff, arriving at the Grotta di Nettuno is unforgettable. Enter the immense, cathedral-like grotto and it really is as though the forces of Neptune, god of the sea, have been at work.' http://www.lonelyplanet.com/italy/sardinia/alghero/sights/cave/grotta-di-nettuno
Neptune's Grotto is a stalactite cave near the town of Alghero on the island of Sardinia, Italy. The cave was discovered by local fishermen in the 18th century and has since developed into a popular tourist attraction.