Laos officially the Lao People's Democratic Republic is a socialist state and the only landlocked country in Southeast Asia at the heart of the Indochinese peninsula, bordered by Myanmar (Burma) and China to the northwest, Vietnam to the east, Cambodia to the southwest, and Thailand to the west and southwest. An ancient human skull was recovered from the Tam Pa Ling Cave in the Annamite Mountains in northern Laos; the skull is at least 46,000 years old, making it the oldest modern human fossil found to date in Southeast Asia. Laos traces its history to the kingdom of Lan Xang (Million Elephants), which was founded in the 14th century by a Lao prince Fa Ngum who with 10,000 Khmer troops, took over Vientiane. In 1975 the Pathet Lao, along with the Vietnam People's Army, and backed by the Soviet Union, overthrew the royalist Lao government, forcing King Savang Vatthana to abdicate on 2 December 1975. He later died in prison. Between 20,000 and 62,000 Laotians died during the Civil War. Laos has a mostly tropical savanna climate. A tropical monsoon and humid sub-tropical climate also occurs in places.