Rural migration into the cities and construction of many settlements were made on an unordered fashion; many colonias or boroughs were settled almost overnight, so basic services like electricity, water, social services and security wouldn't be available until several years later, or never at all. This is known as spatial inequality.One example of such effect is Mexico City: rural migration into the city caused a rise in population from 3 million in 1950, to 5.1 million in 1960, to 8.7 million in 1970. Nowadays it has a population of over 21.16 million. Due to spatial inequality, many people settle on the outskirts of the city, on cardboard shantytowns known as ciudades perdidas.