Gibraltar is a British overseas territory located near the southernmost tip of the Iberian Peninsula overlooking the Strait of Gibraltar. The territory shares a border with Spain to the north. Gibraltar has historically been an important base for the British Armed Forces and is the site of a Royal Navy base. The territory covers 6.543 kilometres, shares a one and a half kilometre land border with Spain and has 12 kilometres of shoreline. There are two coasts of Gibraltar – the East Side, which contains the settlements of Sandy Bay and Catalan Bay, and the West Side, where the vast majority of the population lives. Its terrain consists of the 426 metre high Rock of Gibraltar and the narrow coastal lowland surrounding it. The Rock itself is made of limestone and contains many kilometres of tunnelled roads, most of which are operated by the military and closed to the public. Most of its upper area is covered by a nature reserve, which is home to around 230 Barbary Macaques, commonly known as 'apes', the only wild monkeys found in Europe. They sometimes visit the town area. Recent genetic studies and historical documents point to their presence on the Rock before its capture by the British. A superstition analogous to that of the ravens at the Tower of London states that if the monkeys ever leave, so will the British.