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Mexico City is located near the southern end of the plateau of AnAhuac, at an altitude of c.7,800 ft (2,380 m). The horizons of the city are almost obscured by mountain barriers, and the peaks of PopocatEpetl and Iztaccihuatl are not far off. The climate is cool and dry. Much of Mexico City's surrounding valley is a lake basin with no outlet, and in the past during the rainy seasons, mountain runoff swelled the lakes.
From the time when the Aztec capital of TenochtitlAn stood on an island in Lake Texcoco : now the heart of the metropolis : measures have been taken to protect the city and provide for expansion by draining Texcoco and the other lakes, Chalco and Xochimilco. In the 17th cent. the Spanish viceroys, notably Louis de Velasco, the younger, initiated important works. In 1900 a central canal was completed that reached to the headwaters of the PAnuco River. The Caracol [Span.,=snail], a 12-mi (19-km) spiral canal fed in turn by longitudinal canals begun in 1936, acts as an evaporating basin, from which valuable minerals are taken.
Drainage and artesian wells have lowered the water table so that the surface crust, formerly supported by subsoil water, can no longer sustain the city's heavier buildings, which are sinking some 4 to 12 in. (10.2–30 cm) a year. Some of Mexico's finest buildings have been damaged, among them the old cathedral (begun in 1553 on the site of an Aztec temple) and the Palace of Fine Arts. Modern office buildings have been shored up with pilings.
In addition to being built on soft subsoil, the city is located in a region of high seismic activity. Earthquakes in 1957 and 1985 caused substantial damage. Overcrowding has also become a major problem in Mexico City, and traffic concentrations, combined with the surrounding valley's atmospheric conditions and PopocatEpetl's sulfur dioxide emissions, have resulted in heavy air pollution.
Measures have been taken to attack the pollution problem, and some progress has been made. Since 1989 automobiles have been required to stay off the roads one business day a week. The city's buses have been completely replaced, many major industries have had to convert to low-sulfur fuels, and the government closed the oil refinery.
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