Ethiopia and Eritrea follow the Ethiopian calendar, which is approximately seven years and three months behind the Gregorian, and write with the ancient Ge'ez script, one of the oldest alphabets still in use in the world. Ethiopia is a multilingual nation, with around 80 ethnolinguistic groups, the four largest of which are the Oromo, Amhara, Somali and Tigrayans. Most people in the country speak Afroasiatic languages of the Cushitic or Semitic branches. Additionally, Omotic languages are spoken by ethnic minority groups inhabiting the southern regions. Nilo-Saharan languages are also spoken by the nation's Nilotic ethnic minorities. Oromo is the most populous language by native speakers, while Amharic is the most populous by number of total speakers. Ge'ez remains important as a liturgical language for both the Ethiopian Orthodox Tewahedo Church and the Eritrean Orthodox Tewahedo Church and for the Beta Israel. A majority of the population adheres to Christianity (mainly the Ethiopian Orthodox Tewahedo Church and P'ent'ay), and the historical Kingdom of Aksum was one of the first states to officially adopt the religion. A third follow Islam, primarily Sunni. The country is the site of the Islamic Migration to Abyssinia and the oldest Muslim settlement in Africa, at Negash. A substantial population of Ethiopian Jews, known as Beta Israel, also resided in Ethiopia until the 1980s. The nation is a land of geographical contrasts, ranging from the vast fertile west, with its forests and numerous rivers, to the world's hottest settlement of Dallol in its north. The Ethiopian Highlands are the largest continuous mountain ranges in Africa, and the Sof Omar Caves contains the largest cave on the continent. Ethiopia also has the second-largest number of UNESCO World Heritage Sites in Africa. The sovereign state is a founding member of the UN, the Group of 24 (G-24), the Non-Aligned Movement, the G77 and the Organisation of African Unity. Addis Ababa serves as the headquarters of the African Union, the Pan African Chamber of Commerce and Industry, the United Nations Economic Commission for Africa, the African Standby Force and many of the global NGOs focused on Africa.