page_title: Greater Mexico City
section_title: Geography and environment
hierarchical_section_title: Greater Mexico City / Geography and environment
caption_attribution_description: Español: Mapa comparativo de la superficie del lago de Texcoco y la ciudad de México
context_page_description: Greater Mexico City refers to the conurbation around Mexico City, officially called Mexico City Metropolitan Area, constituted by Mexico City itself composed of 16 Municipalities—and 41 adjacent municipalities of the states of Mexico and Hidalgo. For normative purposes, however, Greater Mexico City most commonly refers to the Metropolitan Area of the Valley of Mexico an agglomeration that incorporates 18 additional municipalities. As of 2018 an estimated 21,581,000 people lived in Greater Mexico City, making it the largest metropolitan area in North America. It is surrounded by thin strips of highlands which separate it from other adjacent metropolitan areas, of which the biggest are Puebla, Toluca, and Cuernavaca-Cuautla, and together with which it makes up the Mexico City megalopolis.
  Since the 1940s there have been different proposals to establish the limits of the growing conurbation of Mexico City, and different definitions were used unofficially as the city continued to grow.
context_section_description: Greater Mexico City spreads over the valley of Mexico, also called the valley of Anáhuac, a 9,560 km² (3,691 sq mi) valley that lies at an average of 2,240 m (7,349 ft) above sea level. Originally, a system of interconnected lakes occupied a large area of the valley, of which Lake Texcoco was the largest. Mexico City was built on the island of Tenochtitlan in the middle of the lake. During conquest of Mexico the dikes that protected the city from recurrent floods were destroyed and colonial authorities preferred to drain the water of the lake, which was, for the most part, shallow. In 1900 president Porfirio Díaz inaugurated the Valley's System of Drainage that hinders the growth of water bodies in the valley (and prevents floods). The basin of the valley of Mexico was thus integrated artificially to the Moctezuma river basin which connects to the Pánuco River. The last remnants of the system of lakes are found in the boroughs of Xochimilco and Tláhuac, and in the municipality of Atenco.
  The valley of Mexico is surrounded by mountains on all four sides creating a basin with only one small opening at the north, trapping all exhaust emissions of the city. At the southern part of the basin the mountain range reaches an altitude of 3,952 m (12,965 ft) above sea level; and to the east the volcanoes reach an altitude of more than 5,000 m (16,000 ft). The region receives anti-cyclonic systems, producing weak winds that do not allow for the dispersion of accumulated air pollutants, produced by the 50,000 industries operating in Greater Mexico City and the 4 million vehicles circulating in its roads and highways.
  There are several environmental programs in operation in all municipalities of Greater Mexico City. One of them is Hoy No Circula (known in English as "One Day without a Car"), whereby some vehicles with certain ending numbers on their license plates are not allowed to circulate on certain days in an attempt to cut down on pollution and traffic congestion. The program groups vehicles by their ending license plate digits, and every weekday vehicles having any of the day's two "hoy no circula" digits are banned from circulating. For instance, on Fridays, vehicles with plates ending in 9 or 0 may not drive. This program is controversial since it has resulted in households buying additional vehicles, either new cars for better-off houses, or very old cheap—and thus more polluting—vehicles. Moreover, newer vehicles are exempt from complying with the program—in that they are manufactured with stricter pollution-reduction equipment—a move said to have been pushed by auto makers to boost sales of new vehicles.
  Other environmental programs include the IMECA (Índice Metropolitano de la Calidad del Aire, "Metropolitan Index of Air Quality") a real-time monitoring of the concentrations of several pollutants on the atmosphere of the valley of Mexico. If the IMECA values reach a critical level, an environmental contingency is declared whereby Hoy No Circula is extended to two days per week, industrial activities are reduced, certain gas power plants shut down, and elementary school entry hours are changed. There has been a decrease in the number of environmental contingencies since the 1990s (due, among other reasons, to the implementation of industrial controls and to the relocation of some factories), from more than 5 to only one or zero a year in the last few years.
caption: Comparative map of the original extent of the system of lakes and the current extent of today's urban area
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