that enclosed the valley, into the River Montezuma, which empties eventually into the Gulf of Mexico. A great tunnel was commenced, in 1607, with 1500 Indians, and completed within a year, its length being more than 6,600 metres (21,650 feet). The falling in of the tunnel was the cause of the great inundations that submerged the city, and attempts were made to convert it into a trench; but this latter undertaking was not finished until 1789, nearly two centuries after its commencement.
The great trench is from 30 to 160 feet in depth, and in some places 300 feet broad, and is known as the Desagüe de Huehuetoca, or the Tajo (Cut) de Nochistongo. Instead of carrying away the waters of the lower lakes, the great canal only drained Zumpango and a river which was diverted into it, leaving Tezcoco and Chalco unaffected directly by the drainage. It, however, relieved the city from apprehension regarding the danger that would have resulted from a sudden overflow of the upper lake into Tezcoco; and by taking away the main tribu-