The combined length of the country's land boundaries is 4,639 km (2,883 mi) and its coastline (excluding islands) is 3,444 km (2,140 mi). The topography consists of hills and densely forested mountains, with level land covering no more than 20%. Mountains account for 40% of the area, with smaller hills accounting for 40% and tropical forests 42%.
The northern part of the country consists mostly of highlands and the Red River Delta. Phan Xi Păng, located in Lào Cai province, is the highest mountain in Vietnam at 3,143 m (10,312 ft). The south is divided into coastal lowlands, Annamite Chain peaks, extensive forests, and poor soil. Comprising five relatively flat plateaus of basalt soil, the highlands account for 16% of the country's arable land and 22% of its total forested land.
The delta of the Red River (also known as the Sông Hồng), a flat, triangular region of 15,000 km2 (5,792 sq mi), is smaller but more intensely developed and more densely populated than the Mekong River Delta. Once an inlet of the Gulf of Tonkin, it has been filled in by the enormous alluvial deposits of the rivers over a period of millennia, and it advances one 100 meters (328.1 ft) into the Gulf annually. The Mekong delta, covering about 40,000 km2 (15,444 sq mi), is a low-level plain no more than 3 meters (9.8 ft) above sea level at any point and criss-crossed by a maze of canals and rivers. So much sediment is carried by the Mekong's various branches and tributaries that the delta advances 60 to 80 meters (196.9 to 262.5 ft) into the sea every year.