Mexico city primarily rests on what was Lake Texcoco. Seismic activity is frequent here. Lake Texcoco was drained starting from the 17th century. Although none of the lake waters remain, the city rests on the lake bed's heavily saturated clay. This soft base is collapsing due to the over-extraction of groundwater, called groundwater-related subsidence. Since the beginning of the 20th century the city has sunk as much as nine metres (30 feet) in some areas. This sinking is causing problems with runoff and wastewater management, leading to flooding problems, especially during the rainy season. The entire lake bed is now paved over and most of the city's remaining forested areas lie in the southern boroughs of Milpa Alta, Tlalpan and Xochimilco. COUNT 5
QSN1 When did draining of Lake Texcoco begin? ANS1 17th century QSN2 How much water is there, approximately, in the lake Mexico City is on? ANS2 none QSN3 How much has the city fallen? ANS3 as much as nine metres (30 feet) QSN4 What does Mexico City rest on? ANS4 Lake Texcoco QSN5 Where are most of the trees located in Mexico City? ANS5 southern boroughs