Mexico City is served by the Sistema de Transporte Colectivo, a 225.9 km (140 mi) metro system, which is the largest in Latin America. The first portions were opened in 1969 and it has expanded to 12 lines with 195 stations. The metro is one of the busiest in the world transporting approximately 4.5 million people every day, surpassed only by subway lines in Moscow (7.5 million), Tokyo (5.9 million), and New York City (5.1 million). It is heavily subsidized, and has some of the lowest fares in the world, each trip costing 5.00 pesos from 05:00 am to midnight. Several stations display pre-Columbian artifacts and architecture that were discovered during the metro's construction.[citation needed] However, the metro covers less than half of the total urban area. The Metro stations are also differentiated by the use of icons and glyphs which were proposed for people who could not read. The specific icons were developed based on historical (characters, sites, pre-Hispanic motifs), linguistic, symbolic (glyphs) or location references and has being emulated in further transportations alternatives in the City and in other Mexican cities. Mexico City is the only city in the world to use the icon reference and has become a popular culture trademark for the city.
Try to answer this question if possible (otherwise reply "unanswerable"): When was the Mexico City metro system first opened?
1969