The Valley of Mexico is one of several depressions which came into being when the Mesa Central of Mexico rose unevenly from a level, which may have been under the surface of the ocean. Its center is now a practically horizontal floor, the top of an enormous mass of volcanic substance and sediments, which have gradually filled up the depression. The valley has no natural outlet, and there is no sure geologic evidence that it has ever had one. In the 17th century an artificial outlet was produced for part of the water entering the valley, when the waters of the Cuauhtitlan River were directed through a tunnel perforating the northern mountain range at Nochistongo. The tunnel soon broke down and was replaced by an open cut, literally an artificial valley, a gigantic engineering job, which, with many interruptions, went on for one and a half century. In 1900 the tunnel at Tequezquiac was opened, which in connection with the Gran Canal running from Mexico City to Zumpango permits the complete draining of the valley. These works have completely changed the physical aspect of the landscape and the living conditions of the inhabitants. Today only shallow sheets of water in the lowest parts of the valley reminds one of the great lakes of Aztec times. The habits and activities of a lacustrine type of life are rapidly disappearing.