Thừa Thiên–Huế Province borders Quảng Trị Province to the north, the city of Đà Nẵng to the east, Quảng Nam Province to the south, and the Savannakhet, Salavan and Kekong provinces of Laos to the west. The Perfume River (called Sông Hương or Hương Giang in Vietnamese) passes through the province. The province also accommodates the Tam Giang-Cầu Hai Lagoon, the largest lagoon in Southeast Asia, which has 68 km in length with 22,000ha of water surface. The province comprises four different zones: a mountainous area, hills, plains and lagoons separated from the sea by sandbanks. It has 128 km of beaches. The mountains, covering more than half the total surface of the province, are along the west and southwest border of the province, their height varying from 500 to 1480 metres. The hills are lower, between 20 to 200 metres, with some points at 400 metres, and occupy about a third of the province's area, between the mountains and the plains. The plains account for about a tenth of the surface area, with a height of only up to 20 metres above sea level. Between the hills are the lagoons which occupy the remaining 5% of the province's surface area.[3]