Sunlight, passing through an underwater cavity and shining through the seawater, creates the magical blue reflection that illuminates the cavern. 

VIDEO: Trip into The Blue Grotto from Marina Grande. 

 

Before you go, you should be aware that you can't always visit the Blue Grotto. The entrance is barely large enough to accommodate the small rowboats. During adverse weather conditions, waves could cause the gunwales of the small boats to smack up against the roof of the opening. It's just 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 for which it is so famous.

The Blue Grotto can be reached by motorboat from the port of Marina Grande. As you can see in my video, when you arrive at the Blue Grotto, you transfer to a small wooden rowboat. Because there is no headroom, you have to lie back almost in the bottom of the rowboat to clear the entranceway. Once inside, the grotto is roomy and you can sit upright to enjoy the view.

When the weather is fine, waterborne traffic jams can occur outside the grotto. The rowboats jostle for positions to enter the cave in single file, while the larger, motorized boats from the marina jostle for the attention of the oarsmen. Before entering the oarsman pays the entry fee just outside the entrance. (in 2010, 11.5 Euros per person, regardless of the number of visitors in the boat) Thanks to(Wikipedia)

 Enjoy the video!