Southern Vietnam is divided into coastal lowlands, the mountains of the Annamite Range, and extensive forests. 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 soil in much of the southern part of Vietnam is relatively low in nutrients as a result of intense cultivation. Several minor earthquakes have been recorded in the past. Most have occurred near the northern Vietnamese border in the provinces of Điện Biên, Lào Cai and Sơn La, while some have been recorded offshore of the central part of the country. The northern part of the country consists mostly of highlands and the Red River Delta. Fansipan (also known as Phan Xi Păng), which is located in Lào Cai Province, is the highest mountain in Vietnam, standing  high. From north to south Vietnam, the country also has numerous islands; Phú Quốc is the largest. The Hang Sơn Đoòng Cave is considered the largest known cave passage in the world since its discovery in 2009. The Ba Bể Lake and Mekong River are the largest lake and longest river in the country.