Baron's Island

 Major Island (Isla Mayor), also called Baron's Island (Isla del Barón), is a Spanish island located almost in the center of the Mar Menor in the municipality of San Javier in the Region of Murcia and, as its name suggests, it is the largest island in that sea, as well as the entire Region of Murcia (93.8 hectares). It is an extinct volcanic cone that gives the island its characteristic conical shape (104 meters high) and round perimeter. The island is privately owned. The name of Isla del Barón comes from the Barón de Benifayó, who built a neo-Mudejar style palace on this island, known as the Baron's house. (Source: Wikipedia)

Every island at Mar Menor were volcanos. Mar Menor is the largest salt lagoon in Europe.

A very interesting story about that place and its owners you can find here (it was also visited by Ava Gadner). In the following link you can find the text in English.