The ancestors of Cambodia had an empire called Angkor centered in the northwest of present-day Cambodia. The Angkor civilization is the world's largest pre-industrial civilization. One of the buildings from Angkor is a Hindu/Buddhist temple called Angkor Wat which is the world's largest religious structure. This empire was later destroyed due to ecological and environmental problems as well as failing infrastructure. Theravada Buddhism came to the country in the 13th century via monks from Sri Lanka. Since then, Buddhism has been the official religion.