Northwestern Cambodia includes the provinces of Banteay Meanchey, Oddar Meanchey, Preah Vihear, Siem Reap and the part of Stoeng Treng to the west of the Mekong. Much of the southern part of the region is very flat and fertile. The north is dominated by the Dangrek Mountains which form the natural barrier between Cambodia and Thailand. The magnificent ruins of several ancient Khmer cities at Angkor Archaeological Park cover some 400 sq km and is one of the world's great monuments of the past. These monuments include the famous Temple of Angkor Wat and, at Angkor Thom, the Bayon Temple with its countless sculptural decorations. Angkor was declared a UNESCO World Heritage site in 1992 and UNESCO has set up a wide-ranging program to safeguard this symbolic site and its surroundings. Angkor itself has no accommodation and few facilities. The nearby town of Siem Reap is the tourist hub for the area. Also within the region is Tonle Sap, the largest freshwater lake in southeast Asia.