Mexico City Metro) was created in order to be in charge of expanding the STC Metro within the metropolitan area of Mexico City. Works began with the expansion of Line 3 towards the north from Tlatelolco to La Raza in 1978 and to the current terminal Indios Verdes in 1979, and towards the south from Hospital General to Centro Médico in 1980 and to Zapata months later. Construction of lines 4 and 5 was begun and completed on 26 May – 30 August 1982, respectively; the former from Martín Carrera to Santa Anita and the latter from Politécnico to Pantitlán. Line 4 was the first STC Metro line built as an elevated track, owing to the lower density of big buildings. Third stage (1983–1985), and the 1985 earthquake This construction stage took place from the beginning of 1983 through the end of 1985. Lines 1, 2 and 3 were expanded to their current lengths, and new lines 6 and 7 were built. The length of the network was increased by and the number of stations to 105. Line 3's route was expanded from Zapata station to Universidad station on 30 August 1983. Line 1 was expanded from Zaragoza to the current terminal Pantitlán, and line 2 from Tacuba to the current terminal Cuatro Caminos. These last two were both inaugurated on 22 August 1984. Line 6's route first ran from El Rosario to Instituto del Petróleo; Line 7 was opened from Tacuba to Barranca del Muerto and runs along the foot of the Sierra de las Cruces mountain range that surrounds the Valley of Mexico at its west side, outside of the ancient lake zone. This made it possible for Line 7 to be built as a deep-bore tunnel. On the morning of 19 September 1985, an 8.1 Richter magnitude earthquake struck Mexico City. Many buildings as well as streets were left with major damage making transportation on the ground difficult, but the STC Metro was not damaged because a rectangular structure had been used instead of arches, making it resistant to earthquakes, thus proving to be a safe means of transportation in a time of crisis. On the day of the quake, the Metro stopped service and completely shut down for fear of electrocution. This caused people to get out of the tunnels from wherever they were and onto the street to try to get where they were going.