The twenty-fourth most populous city in France, Aix-en-Provence - a sub-prefecture of the Bouches-du-Rhône department - has a little more than 143,000 inhabitants and forms, with Marseille, the third-largest urban area in the country with some 1,756,300 inhabitants. Spread over 46,204 acres (18,608 ha) from 239 to 1,676 ft (73 to 511 m) above sea level, it has a population density of 769 inhabitants per square kilometer. The population of Aix-en-Provence particularly developed in the early 1920s, and especially from the 1960s and 1970s, and has since then experienced a steady and sustained increase.