The oldest sanctuary of Poseidon is located in Poseidi, in the area that in antiquity was called ancient Mendi.
The temple was used for more than 1000 years, and there are references about it by Thucydides and 14th century Athonite documents. It was probably built by the Eretrians, who colonized Mendi and had Poseidon as their patron.
The excavations brought to light four large buildings, which are the main temple, two buildings on either side of the temple and an arched building in the eastern part of it. The latter, which is also the oldest, dates back to the protogeometric period (11th-10th century BC).
The floor was made out of clay and the walls were built with large cobblestones.