thumb|270px|Painting by [[Jakob Alt, 1835–36]]

The Blue Grotto () is a sea cave on the coast of the island of Capri, southern Italy. Sunlight shining through an underwater cavity is reflected back upward through the seawater below the cavern, giving the water a blue glow that illuminates the cavern. The cave extends some into the cliff at the surface, and is approximately deep, with a sandy bottom.

Access

The cave is long and wide. The entry is wide and roughly high at low tides, making safe access possible only when tides are low and the sea is calm. To enter the grotto, visitors must lie flat on the bottom of a small four-person rowboat. The oarsman then uses a metal chain attached to the cave walls to guide the boat inside the grotto.

In 2011, a visitor suffered a life-altering injury when his neck was broken while entering the cave. The Cooperativa Battellieri Grotta Azzurra initially denied liability, but settled a damages claim. It was concluded that the boatmen had continued entering the cave when the sea conditions were too dangerous. Swimming in the grotto is forbidden.

Colour

thumb|right|A diagram of how daylight illuminates the cave through the underwater opening.

The Blue Grotto is one of several sea caves worldwide that is flooded with a brilliant blue or emerald light. The quality and nature of the colour in each is determined by its unique combination of depth, breadth, water clarity, and light source.

In the case of the Blue Grotto, the light comes from two sources: the narrow arched entranceway, and a large hole somewhere below the above water entrance, separated by a band of rock between one and two meters tall. Since it is further from the surface, much less light passes through the lower opening, but its depth and size allow it to be the grotto water's primary source of illumination.

As light passes through the water into the cave, red reflections are filtered out and only blue light enters the cave. Objects placed in the water of the grotto famously appear silver. This is caused by tiny bubbles, which cover the outside of the object when they are placed underwater. The bubbles cause the light to refract differently than it does from the surrounding water and gives off the silver effect.

Cultural influence

In 1826, German writer August Kopisch and his friend Ernst Fries, a German painter, visited the cave and recorded their visit in the Kopisch's Entdeckung der blauen Grotte auf der Insel Capri in 1838.

A Neapolitan song dating from the 18th century, "la grotta azzurra" refers to this cave and also proves that it was already well known before its rediscovery in 1826.

In 1842, Danish choreographer August Bournonville set the second act of his ballet Napoli in the Blue Grotto. In this fantastic tale, Golfo, the demon who rules the Blue Grotto, transforms the ballet's heroine, Teresina, into a Naiad.

Mark Twain visited the Blue Grotto in 1867 and recorded his thoughts in his book The Innocents Abroad.

The grotto is highlighted in the 1953 Newbery Honor book Red Sails to Capri by Ann Weil.

In Alberto Moravia's 1954 novel Il disprezzo (Contempt), a vision appears to the protagonist when, under heavy mental stress, he visits the cave alone.

References

  • The Virtual Cave: Seacaves