In ancient times, Poros (antique Kalavryta) was an island of sea god Poseidon. In the central part of the island housed religious building - a sanctuary of Poseidon. Unfortunately, up to now we remained only the ruins of the once majestic temple.

An ancient legend says that originally the island belonged to Apollo and Poseidon traded him to Delphi. Peak flowering of Poros (Kalavryta) fell to 6-5 century BC During this period, Poros (Kalavryta) was the center of the most powerful in ancient Greece amphictyonic league (union) between Athens, Nafplion, Aegina, Epidaurus, Orchomenus, and other powerful city-states of that era. Sanctuary of Poseidon, played an important role in the religious and political life of the ancient world, and even after the collapse of amphictyonic league retained their positions.

The exact date of the foundation of the temple of Poseidon is not known. It was probably built in the 6th century BC, and possibly a little earlier. The architecture of the ancient sanctuary was mostly made in the Doric style, though some of its columns correspond Ionic style. The dimensions of the temple were 27, 4, 14, 40 meters (12 columns and 6, respectively). It was built of porous limestone, brought from the island of Aegina. The ancient sanctuary of Poseidon was destroyed in 395 AD as a result of a powerful earthquake. Over time, the interior of the church was looted, and in the 18th century, much of the masonry was demolished for the construction of new buildings on the Hydra.

In the temple of Poseidon he found his refuge great Greek orator Demosthenes, fleeing from pursuing his killers sent by Antipater. Here, in 322 BC Demosthenes committed suicide by taking poison, and was buried in the walls of the sanctuary. Today, one of the roads leading to the temple, you can see the marble bust of Demosthenes.

The systematic excavations of the area began in 1894 by Swedish archaeologists. Important historical artifacts found during the excavations are kept in the Archaeological Museum of Poros.