9.4663°S, 78.2426°W.
Sechin Bajo is a preceramic ceremonial complex in the Casma Valley of north-central Peru, with the earliest construction phase radiocarbon-dated to approximately 3500 BCE — making its sunken circular plaza one of the oldest known monumental architectural elements in the Americas. The site combines stone, adobe, and clay construction across multiple superimposed building phases extending into the Initial Period after 1800 BCE. German and Peruvian excavations directed by Peter Fuchs since 1992 documented a 10-metre diameter plaza of stacked stone slabs buried beneath later platform fills. Sechin Bajo is one of three closely related sites in the immediate area — Cerro Sechin and Sechin Alto — that together form the Sechin Complex, the largest Initial Period monumental concentration on the Peruvian coast. The early plaza predates the carved warrior monoliths of nearby Cerro Sechin by at least a millennium.
The buried circular court remains accessible through a controlled archaeological cut left open by the excavators.