Tiwanaku (often written in its Spanish form Tiahuanaco) is Bolivia's most important archaeological site and one of the most significant in South America. It was the capital of an empire which was at its most powerful between 500-1000AD, covering western Bolivia, northern Chile and southern Peru. It is believed to have been an important pilgrimage site from around 500BC, and an urban site for a considerable period before that. After 1000AD its influence declined dramatically for as yet unknown reasons (a drying of the climate in the area is one theory). It is located on the altiplano near Lake Titicaca around 72km / 45 miles west of La Paz at an altitude of approx 4000m / 13,100ft. It is best known for its impressive carved stone monoliths (including the famous Sun Gate / Puerta del Sol).