Gyeongju was the capital of the ancient kingdom of Silla (57 BC – 935 AD), which ruled about two-thirds of the Korean Peninsula at its height between the 7th and 9th centuries, for close to one thousand years. Later Silla was a prosperous and wealthy country, and its metropolitan capital of Gyeongju was the fourth largest city in the world. A vast number of archaeological sites and cultural properties from this period remain in the city. Gyeongju is often referred to as "the museum without walls". Among such historical treasures, Seokguram grotto, Bulguksa temple, Gyeongju Historic Areas and Yangdong Folk Village are designated as World Heritage Sites by UNESCO. The many major historical sites have helped Gyeongju become one of the most popular tourist destinations in South Korea.