They are the most famous and visited the island, in the Portocristo area. They consist of four caves joined one to the other: the Cave of the French, the Cave of Luis Salvador, the White Cave and the Black Cave. The sea water enters the caves and forms the Martel lake, 117 m long and 30 wide, one of the largest underground lakes in the world. While hiking visitors can also listen to a classical music concert with works by Chopin, Caballero and Offenbach. Along with the concert you can take a short boat trip on the lake, beautifully illuminated with light.