This deforestation began in the mid 19th century by loggers and “chicleros” who tapped trees for sap to make chewing gum. By the 1940s, much of the old growth forest had already been destroyed. Illegal logging is still a serious concern. Twenty one municipalities in Chiapas have significant problems with illegal logging, most of which are in or near the Montes Azules Reserve. While migration of people into the lowland rainforests had been going on since the 1930s, it accelerated in the 1960s and 1970s, as there was high population growth in the highlands areas. The government encouraged people, especially the indigenous, to move to the lowlands and claim lands there. During the 20th century, the population of municipalities in this area, such as Altamirano, Las Margaritas, Ocosingo and Palenque has risen from 11,000 in 1920 to over 376,000 in 2000. In 1990, a World Bank study declared that the following decade would make or break the Lacandon Selva's chances for survival as the rainforest had been "reduced to the minimum size essential for the integrity of its ecosystem." The destruction of the jungle has been such that satellite photos show the Mexico-Guatemalan border where the deforestation on the Mexican side stops.