Boats for the visit to the famous Blue Grotto, Capri, Bay of Naples, Italy, Europe

Snorkelers swim above a blue hole on Palau's barrier reef. This hole formed when sea level was much lower than it is today.

Sunset against limestone grotto islands, Halong Bay, Vietnam, Indochina, Southeast Asia, Asia

Entry to Grotto di Blue Marino, Golfo di Orosei, Ogliastra, Sardinia, Italy

Ras il Bajjada at Blue Grotto, Malta

Front view of a man walking on a pavement, Grotto bay beach hotel, Bermuda

Two armchairs placed near a tree, Grotto bay, Bermuda

Sung Sot Cave, Ha Long Bay, Quang Ninh, Vietnam