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Capri's Sea Cave Glows Brilliantly Blue Thanks To Its Weird Geology

Once visited by Roman emperors and (supposedly) their gods, this sea cave is now a magnet for tourists.


Tom Hale

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist

Blue Grotto on the coast of the island of Capri, Italy

It's easy to see why the Blue Grotto is one of Capri's biggest tourist attractions.

Image credit: takmat71/

With its vibrantly bright water, the Blue Grotto of Capri may look like it’s been backlit with artificial lighting or perhaps even Photoshopped beyond reality. However, rest assured, the radiant waters of this picture-perfect sea cave are totally natural. 

Also known as the Grotta Azzurra, it can be found on the island of Capri on the south side of the Gulf of Naples in Italy. Entered through a small hole in a cliff face, the cave extends some 50 meters (164 feet) and holds water that’s around 150 meters (490 feet) deep.


Daylight doesn’t just enter from the grotto’s entrance, which small rowboats can sail through, but also via another underwater opening found directly under the cave's mouth. Together with the clear quality of the water and the angle of the Sun throughout most of the day, light can blast through this submerged entrance to illuminate the waters from below, creating this beaming blue effect. 

A Swiss illustration showing of how daylight illuminates the cave through the underwater opening.
A Swiss illustration showing how daylight illuminates the cave through the underwater opening.
Image credit: ETH-Bibliothek/Wikimedia Commons (CC BY-SA 4.0)

The grotto has been admired for thousands of years. It’s said to have been the personal swimming hole of Tiberius, the second Roman Emperor who ruled from 14 to 37 CE.

Three statues depicting Neptune and other Roman mythological figures were discovered on the seabed of the cavern in 1964, indicating it may have also been used as a temple in antiquity. 

Several more statue bases were found in 2009 during an archaeological survey. Upon the discovery, researchers commented on how it lined up with an account by the Roman scholar Pliny the Elder (23 to 79 CE) who described the sea cave as a hangout of the demi-god merman Triton.


The cave appears in an 1838 book called Entdeckung der blauen Grotte auf der Insel Capri (translated as Discovery of the Blue Grotto on the Isle of Capri) in which German poet and painter August Kopisch describes his supposed rediscovery of the area in 1826. 

It’s believed this book kick-started the massive interest in the island of Capri among European artists and writers during the 19th century. Even today, the island's sights still attract the rich and famous (although that's pop stars and influencers nowadays, not poets and painters). 

It is possible for tourists to sail into the Blue Grotto and see its radiant waters with their own eyes. However, bear in mind, the island can get pretty packed during the summer months.


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