The water inside the Blue Grotto on Capri looks as if it is lit from below. There are no lamps and no special effects. The glow is natural, and it comes from the shape of the cave.
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The cave has two openings. One is a low gap at sea level, only about a metre high, and small rowing boats use it to get in. The other is a much larger opening under the water, just below the first. Sunlight enters through this underwater gap and rises into the cave from below.
On the way, the sea absorbs most of the red light and leaves mostly blue. The pale sea floor then reflects that blue light around the cavern. This is why the effect is strongest around midday and much weaker on a grey day. Objects in the water can look silver because tiny bubbles cling to their surface.
The cave is about 60 metres long and 14 metres high, and people knew it long before modern tourism. In Roman times it was part of a villa, and under Emperor Tiberius, who ruled from Capri, it served as a sea temple. Statues of sea gods found inside are now in the Casa Rossa museum in Anacapri.
In later centuries, the cave was mostly forgotten, and fear took its place. Local people believed that witches, spirits and monsters lived inside. It is easy to see why. The entrance is low and dark, and the light in the water is strange and unearthly. Boatmen stayed away.
Things changed in 1826, when a fisherman took the German writer August Kopisch and the painter Ernst Fries into the cave. Kopisch described the visit in a book published in 1838. It helped to draw European artists and writers to Capri.
Many were on the Grand Tour. This was a long journey through Europe, mostly Italy, that wealthy young people, especially from Britain, made for months or even years. The grotto became one of its favourite stops.
Even today, entering is not simple. Visitors lie back in small boats while a boatman pulls them through the low entrance with a chain fixed to the rock. When the sea is rough, the cave is closed, and swimming inside is forbidden.