Southern Vietnam is divided into coastal lowlands, the mountains of the Annamite Range, and extensive forests. Comprising five somewhat 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 lower in nutrients as a result of cultivation. Several earthquakes have been recorded in the past. Most have occurred near the northern border, 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, which is located in Lào Cai, is the highest mountain in Vietnam, standing high. From north to south, the country also has various 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.