 However, the distribution of the population is widely uneven.The most densely populated areas are the Po Valley (that accounts for almost a half of the national population) and the metropolitan areas of Rome and Naples, while vast regions such as the Alps and Apennines highlands, the plateaus of Basilicata and the island of Sardinia are very sparsely populated.The population of Italy almost doubled during the 20th century, but the pattern of growth was extremely uneven because of large-scale internal migration from the rural South to the industrial cities of the North, a phenomenon which happened as a consequence of the Italian economic miracle of the 1950–1960s.