Stone tools discovered in Houaphanh and Luang Prabang provinces attest to the presence of prehistoric man in the hunter-gatherer stage in Lao territory from at least 40,000 years ago. Laos is landlocked and lies between Thailand, Myanmar, China, Vietnam and Cambodia. The Mekong River forms the country's western border and the Annamite Mountains form its eastern border with Vietnam. The total land area is approximately 237,000 sq km, an area similar to that of the United Kingdom. The main geographic features of the country are its mountains and the Mekong River valley. The curtural highlights include the UNESCO World Heritage sites of Wat Phou, Oup Mong, the historic city of Luang Prabang area with its many temples and monasteries and the Plain of Jars.