Founded at the beginning of the 9th century CE, the Khmer Empire had been one of the most powerful states in Southeast Asia, covering much of Cambodia, Thailand, Laos, and Southern Vietnam. The jewel in the crown was the capital city of the Khmer – Angkor – containing the largest religious structure ever built, Angkor Wat. The key to this growth was the Khmer mastery of rice cultivation and water management. Khmer cities were ‘hydraulic cities', made up of complex systems of canals and reservoirs called ‘barays', which steered water from the Khulen mountains to the plains of Angkor, ensuring a year-round supply for the population, agriculture, and livestock. Soon, however, the very factors that allowed the empire to rise lead to the abandonment of the Khmer's magnificent temple-cities and their reclaim by the rainforest.