Capri has a blue hole known as one of the world's seven wonders. Blue hole is a limestone terrain, a sea erosion hole, the hole is under the cliff, the hole is very small, the sea surface is only 85 cm high, so you can only swim or take a boat to enter on a calm and sunny day. The blue hole is called the "blue hole" because the sun shines from the narrow hole into the spacious hole, and the blue water reflected through the hole makes the entire sea erosion hole appear a beautiful crystal blue.