The National Marine Park of Alonnisos Northern Sporades was founded by Presidential Decree on May 16, 1992. It was the first of its kind in Greece, and is currently the largest marine protected area in Europe (approximately 2.260 km2). Besides the sea area, the park includes the island of Alonnisos, six smaller islands, as well as 22 uninhabited islets and rocky outcrops.
It is located in the region of the Northern Sporades Islands, in the northern Aegean Sea. Administratively, the Sporades Islands belong to the prefecture of Magnesia. Limestone rock dominates the island surfaces. Its main characteristics are the steep rocky slopes which run down to the sea and the caves, which are an important part of the habitat of the monk seal. Different types of soil are encountered while fresh water sources are scarce.