In antiquity Poros (ancient Kalaureia) was the island of the sea God Poseidon. In the Central part of the island housed a religious building - the sanctuary of Poseidon. Unfortunately, to this day the ruins of the once majestic temple.

An ancient legend says that the island originally belonged to Apollo, and Poseidon traded it for Delphi. The peak of Poros (Calabria) fell to 6-5 century BC during this period Poros (Calauria) was the center of the most powerful in Ancient Greece Amphictyony (Union) between Athens, Nafplion, Aegina, Epidaurus, Orchomenos and other powerful city-States of that era. The sanctuary of Poseidon played an important role in the religious and political life of the ancient world and even after the collapse of Amphictyony maintained its position.

The exact date of Foundation of the temple of Poseidon is not known. Probably it was built in the 6th century BC, and possibly somewhat earlier. The architecture of the ancient sanctuary was mostly designed in the Doric style, though some of its columns correspond to the Ionian style. The dimensions of the temple was 27, 4 at 14, 40 meters (12 and 6 columns respectively). It was built of porous limestone, brought from the island of Aegina. The ancient sanctuary of Poseidon was destroyed in ad 395, the result of a powerful earthquake. Over time, the interior of the temple was plundered, and in the 18th century most of the masonry was dismantled for the construction of new buildings on the Hydra.

In the temple of Poseidon found refuge the great Greek orator Demosthenes order to escape 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 on one of the roads leading to the temple you can see a marble bust of Demosthenes.

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