Sechin Bajo Circular Plaza

9.4700°S, 78.2850°W.

The Sechín Bajo Circular Plaza in the Casma Valley, Áncash Region, Peru — the earliest documented monumental architectural feature in the Americas, with the principal sunken circular plaza radiocarbon-dated to approximately 3500 BCE. The structure is a sunken circular plaza 10–12 metres in diameter cut into the foundation of a platform pyramid, accessed by stone-lined steps on the eastern flank. Excavated by the Free University of Berlin under Peter Fuchs from 1992 onward, with the foundational radiocarbon dates published in 2008. The 3500 BCE date predates the foundation of Caral by approximately 500 years and pushed the chronology of monumental Andean architecture back by half a millennium when first announced. The sunken-plaza-with-platform template that originates here recurs at Caral, Aspero, and across the entire Late Archaic and Initial Period of Andean civilisation — every major early Peruvian ceremonial centre includes a version of this same architectural unit.

The earliest documented monumental structure in the western hemisphere, in an architectural form that defined Peruvian religious building for the next four thousand years.