The name Tanzania is a portmanteau of Tanganyika, the mainland, and Zanzibar, the nearby archipelago in the Indian Ocean. The two united to become the United Republic of Tanzania in 1964. With a surface area of 947,300 square kilometers, Tanzania is comparable in size to Nigeria and is slightly more than twice the size of the U.S. state of California. Tanzania's population of approximately 40.4 million (as of 2007) is the second largest in East Africa, after Ethiopia's. Dar es Salaam, the most populous city, contains approximately 2.7 million people and accounts for most commercial activity. Swahili (or Kiswahili) and English are the two official languages of Tanzania. A large number of local languages are also spoken. In Zanzibar, Arabic is commonly used. Agriculture remains the mainstay of Tanzania's economy, accounting for one-quarter of gross domestic product (GDP) and approximately 80 percent of employment. Tanzania is endowed with mineral and natural resources, including gold, diamonds, and several other precious and semiprecious stones. The blue gemstone tanzanite is found only in Tanzania. Tanzania accounted for almost two percent of world gold production as of 2006. Tanzania has a long history of hosting refugee's fleeing civil wars in nearby countries. As of January 2008, there were more than 380,000 refugees living in Tanzania, predominantly from Burundi and the Democratic Republic of Congo. Tanzania is an up-market tourism destination. The country is endowed with a variety of tourism assets, including seven United Nations Educational, Scientific, and Cultural Organization (UNESCO) world heritage sites and numerous wildlife parks, beach resorts, coral reefs, and spectacular scenic mountain views.