Capri Island has the Blue Grotto, which is known as one of the seven wonders of the world. The Blue Grotto is a limestone terrain and a sea cave. The entrance is under the cliff and is very small. The height above the sea surface is only 85 cm, so you can only enter by swimming or taking a boat on a calm sunny day. The Blue Grotto is called the "Blue Grotto" because the sunlight shines into the spacious cave from the narrow entrance, and is reflected by the blue water in the cave, making the entire sea cave appear a beautiful crystal blue.