Blue Grotto (Capri)
Entrance to the Blue Grotto
The Blue Grotto (Italian: Grotta Azzurra) is a noted sea cave on the coast of the island of Capri, Italy. Sunlight, passing through an underwater cavity and shining through the seawater, creates a blue reflection that illuminates the cavern.
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 color in each cave is determined by the unique lighting conditions in that particular cave.
In the case of the Blue Grotto, the light comes from two sources. One is a small hole in the cave wall, precisely at the waterline, that is a meter and half in diameter. This hole is barely large enough to admit a tiny rowboat, and is used as the entranceway. In photographs taken from within the cave, the above-water half of this hole appears as a spot of brilliant white light. The second source of light is a second hole, with a surface area about ten times as large as the first, which lies directly below the entranceway, separated from it by a bar of rock between one and two meters thick. Much less light, per square meter, is able to enter through the lower opening, but its large size ensures that it is, in practice, the primary source of light.
In part because of the dazzling effect of the light from the above-water opening, it is impossible for a visitor who is in one of the row-boats to identify the shape of the larger hole, the outline of the bar that separates the two holes, or even the nature of the light-source, other than a general awareness that the light is coming up from underneath, and that the water in the cave is more light-filled than the air. A visitor who places a hand in the water can see it "glow" eerily in this light.
This video, taken by a swimmer in the cave, provides a clear underwater view of both openings.
The grotto was known by the Romans, as proved by the antique statues which were found in the Grotto. This discovery, the remains of an ancient landing place and the work on an underground tunnel, create an image of a natural cavern adorned by statues. The grotto was known to the locals under the name of Gradola, after the nearby landing place of Gradola, but it was avoided because it was said to be inhabited by witches and monsters.
The Blue Grotto became a favoured tourist destination in 1830s, after the visit of German writer August Kopisch and his friend Ernst Fries to the cave in 1826 and after the issuing of the book of Kopisch Entdeckung der blauen Grotte auf der Insel Capri in 1838. They were guided to the cave by a local fisherman Angelo Ferraro and during their visit they noticed the presence of Roman structures in the cave. Since then the Blue Grotto has become the emblem of the island of Capri.
Visiting the Blue Grotto 
The Blue Grotto can be reached by motorboat from the port of Marina Grande, by bus from Anacapri, or by taxi. To enter the low opening to the grotto, the tourist transfers from the motorized boat that brought him or her from the port into a small wooden rowboat manned by one of the oarsmen who specialize in ferrying travellers into the cave. Because there is no headroom, visitors must lie on their backs in the bottom of the rowboats as they clear the entranceway. The interior of the grotto is quite spacious, and once inside it is possible to sit upright, until conveyed back out through the same tiny hole.
The grotto cannot be visited during adverse weather conditions, as the entrance is barely large enough to accommodate the small rowboats, and waves can cause the gunwales of the small boats to be smacked upwards against the roof of the opening. As well, bright sunlight is necessary to create enough light to cause the interior of the cave to be lit up in the brilliant blue color for which it is so famous.
When the weather is fine, waterborne traffic jams frequently occur outside the grotto, as the rowboats jostle for positions to enter the cave in single file. Also, the larger, motorized boats from the marina jostle for the attention of the oarsmen at the small rowboats, and once visitors have been transferred to the small boats it is necessary for the oarsman to paddle to another boat, anchored just outside the entrance, where the entry fee (in 2012, 12.5 Euros per person, regardless of the number of visitors in the boat), is paid.
Additionally, it is possible to swim into the cave, though this is best done later in the day when the boats are all gone. There is a stairway that leads down to the water nearby which makes access not very difficult. Beware, if the seas are at all stormy, it is somewhat dangerous as you pass through the small opening, however this also means that this is the best time to enter during normal daylight as the boats cannot enter at all with any sort of large seas.
Cultural influence 
The grotto is highlighted in the 1953 Newbery Medal book, Red Sails To Capri, by Ann Weil.
See also 
Media related to Blue Grotto at Wikimedia Commons