he ultra-conservative Wahhabism religious movement within Sunni Islam has been called "the predominant feature of Saudi culture", with its global spreading largely financed by the oil and gas trade. Saudi Arabia is sometimes called "the Land of the Two Holy Mosques" in reference to Al-Masjid al-Haram (in Mecca), and Al-Masjid an-Nabawi (in Medina), the two holiest places in Islam. The Kingdom has a total population of 28.7 million, of which 20 million are Saudi nationals and 8 million are foreigners.Petroleum was discovered in 1938 and followed up by several other finds in the Shia-majority Eastern Province.[16] Saudi Arabia has since become the world's largest oil producer and exporter, controlling the world's second largest oil reserves, and the sixth largest gas reserves.The kingdom is categorized as a World Bank high-income economy with a high Human Development Index,[18] and is the only Arab country to be part of the G-20 major economies. However, the economy of Saudi Arabia is the least diversified in the Gulf Cooperation Council, lacking any significant service or production sector (apart from the extraction of resources).Following the unification of the kingdoms of by royal decree on 23 September 1932 by its founder, Abdulaziz Al Saud (Ibn Saud). Although this is normally translated as "the Kingdom of Saudi Arabia" in English[32] it literally means "the Saudi Arab kingdom",or "the Arab Saudi Kingdom".