Angkor, the medieval Cambodian capital, was a vast, low-density urban complex covering about 1000 sq km - the most extensive city of its kind in the pre-industrial world. Established in the 9th century it is the location of the largest religious monument on Earth, Angkor Wat, the largest single concentration of shrines and the location of massive, human–built reservoirs. The reservoirs were linked with an enormous infrastructure of canals and embankments used to manage surface water resources. Yet despite the sheer physical enormity of the city, by the 19th century the urban world of Angkor was gone and forest had reclaimed much of the urban landscape. The water management network showed evidence of systemic collapse - some former canals were incised 5-10 m below the Angkorean ground surface, while others were choked with sand.