 The valley has no natural drainage outlet for the waters that flow from the mountainsides, making the city vulnerable to flooding.It was artificially opened through the use of canals and tunnels starting in the 17th century, entirely draining what used to be Lake Texcoco.The rural southern portion of the Federal District and of the Valley of Mexico, in particular the Sierra Chichinautzin is the most important natural recharge zone for the Mexico City Aquifer due to relatively high levels of precipitation and the high permeability of its basalt rock.