Iran has three distinct geographical areas: mountain ranges, interior basins, and lowland plains. The principal mountain ranges include the Zagros Mountains in the west and south, and the Alborz Mountains in the north. Iran's culture is a mix of ancient pre-Islamic culture and Islamic culture. Iranian culture has long been a predominant culture of the Middle East and Central Asia. Iran has the second largest natural gas reserves in the world and the fourth largest proven crude oil reserves. However, economic activity and government revenues are still dependent on oil revenues.