Rwanda is a small, landlocked country in East-Central Africa, located just south of the Equator. Its capital, Kigali, is known for being one of the cleanest and safest cities in Africa. Rwanda is nicknamed 'Le Pays des Mille Collines' (The Land of a Thousand Hills) because of its rugged, mountainous terrain. The country is part of the Great Rift Valley system and features the Virunga Mountains in the northwest, which are home to about a third of the world's remaining mountain gorillas. Lake Kivu, one of the African Great Lakes, forms the border with the Democratic Republic of the Congo. Rwanda's high elevation provides it with a temperate tropical highland climate, with two rainy and two dry seasons. The economy is primarily based on subsistence agriculture, but the country has become a leader in environmental policy, notably being one of the first in the world to ban plastic bags.