The engineers' strike at the El Teniente copper mine of 1973 (b/w photo)

The El Teniente copper deposit is the largest underground mine in the world, with 2,400 km of galleries and 11,000 workers. Located 120 km south of Santiago de Chile at 2,300 meters above sea level, it has been in operation since 1905. Three years before Salvador Allende's Chilean Popular Unity came to power, during the process of "Chileanization of the copper mines" decided in 1967 by Chilean President Eduardo Frei Ruiz-Tagle, the government bought 51% of the shares of the "El Teniente" mine belonging to Braden Copper. This transferred its property rights to the Sociedad Minera d'El Teniente on March 21, 1967. The engineers' strike at the El Teniente copper mine of 1973, at a time when it employed 8,000 to 9,000 workers and 4,500 employees and managers, lasted 74 days. The "march on Santiago" will lead to the coup of September 11, 1974.