The ruins of the Temple of Poseidon in Poros is situated in the northern part of the island, although not much remains of this sanctuary today. Only the foundations and some lower wall sections are visible, and the statues that it once occupied the space, have been pillaged. During the 18th century, much of the structure was dismantled and used to build a monastery on Hydra. It used to be the center of the Kalaureian amphictyony that functioned as a voluntary co-operative for dealing with religious as well as civic issues for city states including Epidauros, Prassies, Hermione, Aigina, Athens and Orhomenos.
Demosthenes, the ancient great orator, came to Poseidon's Temple demanding his right of sanctuary when he was being chased by Philip the King of Macedonia. He killed himself by drinking the poison hemlock in 322 BC.
Archaeological research on the site began in 1894 by the Swedish Archaeological Institute under the direction of archaeologists Samuel Wide and Lennart Kjellberg. In 1997 the Institute resumed a new phase of archaeological fieldwork in the area, the Kalaureia Research Program that continues to date.