The Akapana Pyramid, Tiwanaku, Bolivia. od ER's Eyes

Climbed the hill up to Tiwanaku’s most outstanding structure, the partially excavated Akapana pyramid, which was built on an existing geological formation. At its base this roughly square, 16m hill covers a surface area of about 200 sq meters. In the center of its flat summit is an oval-shaped sunken area, which some sources attribute to early, haphazard, Spanish excavation. The presence of a stone drain in the center, however, has led some archaeologists to believe it was used for water storage. Recent findings include craniums, assumed to be war trophies, leading some archaeologists to believe the pyramid may have been a ceremonial temple. Others think it was used for the study of astronomy.