For the most part, Mexico City is a low, sprawling mass of grey and brown buildings set along a rectangular pattern of narrow streets. The pattern is broken by several broad boulevards lined with modern high-rise apartment and commercial buildings, by several large open plazas, and by numerous forested parks. The parks are popular with residents, who meet to talk, stroll, and enjoy the city's dry, spring-like climate. The centre of the city has historically been the Zócalo, or Plaza of the Constitution, which occupies the site of the central square of the Aztec city Tenochtitlán, flanked by the massive Baroque National Cathedral (begun 1573, completed 1675), the Municipal Palace (1720), and the National Palace (1792), containing the office of the president and the senate. From the Zócalo the major avenue extends north to the Plaza of the Three Cultures, which has Aztec, Spanish colonial, and modern structures, and south to the sprawling Chapultepec Park, which contains several museums, a zoo, and Chapultepec Castle, the former presidential residence. The city's outward growth has created numerous suburbs and neighbourhoods of great diversity, ranging from the elegant residential area of Pedregal, with its modernistic architecture, to the crowded squatter settlement of Netzahualcóyotl, located on the dry bed of Lake Texcoco. The rapid growth of Mexico City has created several problems, including serious air pollution, an increasingly inadequate water supply, and the subsidence, by as much as 6 m (20 ft), of parts of the downtown area into the soft lake deposits that underlie much of the city, damaging buildings and disrupting some water and sewerage lines. Supplemental water is now obtained from distant sources outside the valley, and modern multistoried buildings are built on huge steel and concrete drums to prevent their sinking. By the 1920s, plans for the urbanization of Mexico City had been initiated. Industrialization increased as mills and factories spread throughout the city. Slum-clearance and housing-development programmes were initiated. Between 1930 and 1950, the population more than doubled. In 1985 a devastating earthquake caused severe damage, leaving nearly 30,000 homeless and thousands more dead.