The destruction of the last of these canals began in the 1950s. At that time, groundwater pumping under the city center was causing severe subsidence. These wells were closed and new ones dug in Xochimilco and other southern boroughs. High rates of extraction have had the same effect on water tables and canals began to dry. Since then reclaimed wastewater has been recycled to flow into the Xochimilco canals to supplement water from natural sources. However, this water is not potable, containing bacteria and heavy metals and the canals still receive untreated wastewater and other pollution Another major problem, especially in the past two decades has been the population explosion of Mexico City, pushing urban sprawl further south into formerly rural areas of the Federal District. This prompted authorities to seek World Heritage Site status for the canals and the pre-Hispanic chinampa fields to provide them with more environmental protection. This was granted in 1987, but these same major environmental problems still exist. A 2006 study by UNESCO and Universidad Autónoma Metropolitana has shown that there are still very high levels of pollution (both garbage and fecal matter) in the canals and there still a rapid rate of deterioration 2,000 hectares of protected area. UNESCO has placed the most blame for the problems on the proliferation of illegal human settlements in the protected zone. Each year the borough loses six hectares of former lakebed to illegal settlements. According to the borough, about 90,000 people in Xochimilco live in illegal settlements, such as those in ecological zones, and 33,804 families live illegally on the chinampas. The most problematic are those closest to the canals, which cause the most pollution. The area is also sinking per year due to over pumping of groundwater, and canals are being filled in illegally. The deterioration is happening so fast, that UNESCO has threatened to pull Xochimilco World Heritage Site status.