Nigeria has four main regions: a humid coastal belt, hilly tropical rainforests, two large plateaus - the Jos and the Biu and swaths of savannah grasslands that descend into semi-desert. Nigeria is home to 148 million people, with 50% living in urban areas, making it the most populous country. Nigeria is a federation of 36 states and is home to over 400 ethnic groups; the three main ones are the Hausa-Fulani, Muslim, the Yoruba who follow Christian and Islam faiths, and the Igbo, most of whom are Christians. Agriculture makes up about 40% of the country's GDP. Oil was discovered in the country in 1956, which by 2008 accounted for 90% of the country's export earnings. Much of the oil wealth was not distributed widely, with military leaders at the top stealing the wealth. Nigeria achieved independence in 1960, and ethnic conflicts over shares of the oil revenue have occurred. In 1967 the Eastern region led by a military governor attempted to secede from Nigeria - a civil war followed that spanned 2.5 years. There have been a series of military coups since. In 1993 General Sani Abacha led a violent military regime; he came to power following an annulment of the presidential election. Abacha imprisoned the selected president, Chief Abiola, and launched violent attacks on the opposition. He died in 1998. A presidential election in 1999 followed with Olusegun Obasanjo and the People's Democratic Party (PDP) taking office. He was re-elected in 2003. In 2007 Umaru Yar'Adua was hand-selected to take office, and in 2010 following his health issues, he handed power to his vice-president Goodluck Jonathan. Jonathan was elected in 2011; violence broke out in the north, with 800 people dying as a result. The most significant contemporary threat in contemporary Nigeria is Boko Haram - an Islamist fundamentalist group that aims to overthrow the state and impose Islamic rule. Boko Haram was founded in 2002 and has carried out a series of violent acts, including suicide bombings and abductions. In May 2014, Boko Haram abducted 300 schoolgirls from the town of Chibok. By 2015 Boko Haram has killed thousands of Nigerians and maintained control over a region in the north-east.