Poseidon was one of the greatest Gods of ancient Arcadia. His name literally means ‘husband of earth’. And he was indeed the partner of the Great Mother Demetra. He had many names, they don’t all relate to his impetus though. They called him Oletira, Mochlotira and Ippio. Originally God of earthquakes and later God of water, he has a strong connection with the famous horses since these creatures are very sensitive to the subterranean currents and its forces. One of the most important temples,situated south east of Mantinia, is dedicated to the Great God. It is close to Arnis spring, where he was born. This significant temple was forbidden to lay people and only once a year a most sacred priest celebrated the secret honors to Poseidon far away from the eyes of the faithful. The building is much afflicted. It was built by the well known Viotous architects, Trofonios and Agamidis, who were renowned as builders for the immortals. However in the Roman times it became a ruin. Adrianos took care of the complete renovation when he visited Mantinia in 130 BC. The temple was re-discovered by the French archaeologist Fougeres who toiled so hard on the Arcadian land. Later the distinguished Greek archaeologist Th. Spiropoulos took on the work. They discovered the base of the sanctuary, the complex of sanctuary rooms, the bases of the polythyron’ spillars.From the box-shaped Byzantine tomb they concluded that the temple had been used for private purposes by a distinguished landowner. The ancient water supply reservoir and valuable inscriptions and architectural materials also came to light.