Mt. Vesuvius is the only active volcano in continental Europe, the most populated (20 municipalities with a total of 400,000 residents) and is also the most extensively studied volcano on Earth. The current shape of the volcano is the result of the continual alternation between "explosive" type eruptions, which have produced pyroclastic deposits, and "effusive" type ones, which instead have produced lava; the resulting volcanic structure is defined as volcanic stratum. The earliest eruptions, which took place between 25,000 and 17,000 years ago, partly destroyed the most ancient volcano, Mt. Somma, within which the Gran Cono of Mt. Vesuvius later formed. The two structures as a whole are known as the Somma-Vesuvius volcanic complex and are a typical example of a fence layer volcano. The highest point of Mt. Vesuvius reaches 1,282 metres above sea level. The current crater has a diameter of 650 metres and a depth of 230 metres. At the base of the crater there are a number of eruptive openings which discharged many of the streams of lava from 1631 up until 1944, the year of the last eruption. Today there are small fumaroles on the Gran Cono of Mt. Vesuvius, a sign of its dormant state. Mt. Somma reaches a height of 1,132 metres above sea level. with Punta Nasone, and has a base diameter of approximately 15 km. The "fence" of Mt. Somma is well preserved on its northern part and the old crater edge has a succession of peaks known as Cognoli. On the external slope meteorological events over thousands of years have created numerous ravines. The southern enclosure, however, has now disappeared completely and in its place there is a flat area, called Piano delle Ginestre. Mt. Vesuvius and Mt. Somma are separated by a depression known as Valle del Gigante, itself divided into Atrio del Cavallo to the west and Valle dell'Inferno to the east.