 After heavy floods in 1950 and 1951 it became clear that the Grand Canal could not protect the city any more from flooding and a deep drainage system ("Drenaje profundo") was proposed for the first time.Studies on the system began in 1959; its construction began in 1967 and it was completed in 1975.It consists of a deep tunnel, the "Emisor Central" with a length of 68 km and a depth of up to 250 m. It today constitutes by far the most important element of the Mexico City drainage system.