Ancient Greek religion was essentially propitiatory in nature, i.e., based on the notion that to avoid misfortune, one must constantly seek the favour of the relevant gods by prayers, gifts and sacrifices. To the ancient Greek, every natural feature, e.g. hill, lake, stream or wood, was controlled by a god. A person about to swim in a river, for example, would say a prayer to the river-god, or make an offering to that god's shrine, to avoid the chance of drowning. The gods were considered immortal and could change shape, become invisible and travel anywhere instantaneously. But in many other respects they were considered similar to humans. They shared the whole range of human emotions, both positive and negative. Thus, in their attitudes towards humans, they could be both benevolent and malicious. Also like humans, they had family and clan hierarchies. They could even mate with humans, and produce demi-gods.
In a maritime country like Greece, the god of the sea occupied a high position in the divine hierarchy. In power, Poseidon was considered second only to Zeus (Jupiter), the supreme god of the Olympian pantheon. His implacable wrath, manifested in the form of storms, was greatly feared by all mariners. In an age without mechanical power, storms very frequently resulted in shipwrecks and drownings.
The temple at Cape Sounion, Attica, therefore, was a venue where mariners, and also entire cities or states, could propitiate Poseidon by making animal sacrifice or leaving gifts.
The temple of Poseidon was constructed in 444–440 BC, over the ruins of a temple dating from the Archaic period. It is perched above the sea at a height of almost 60 metres (200 ft). The design of the temple is a typical hexastyle, i.e., it had a front portico with six columns. Only some columns of the Sounion temple stand today, but when intact it would have closely resembled the contemporary and well-preserved Temple of Hephaestus beneath the Acropolis, which may have been designed by the same architect.
As with all Greek temples, the Poseidon building was rectangular, with a colonnade on all four sides. The total number of original columns was 34: 15 columns still stand today. The columns are of the Doric Order. They were made of locally quarried white marble. They were 6.10 m (20 ft) high, with a diameter of 1 m (3.1 ft) at the base and 79 cm (31 inches) at the top.
At the centre of the temple colonnade would have been the hall of worship (naos), a windowless rectangular room, similar to the partly intact hall at the Temple of Hephaestus. It would have contained, at one end facing the entrance, the cult image, a colossal, ceiling-height (6 metres (20 ft)) bronze statue of Poseidon. Probably covered in gold leaf, it may have resembled a contemporary representation of the god, appropriately found in a shipwreck, shown in the figure above. Poseidon was usually portrayed carrying a trident, the weapon he supposedly used to stir up storms. On the longest day of the year, the sun sets exactly in the middle of the caldera of the island of Patroklos, the extinct volcano that lies offshore, suggesting astrological significance for the siting of the temple. The temple of Poseidon was destroyed in 399 by Emperor Arcadius.
Archaeological excavation of the site in 1906 uncovered numerous artifacts and inscriptions, most notably a marble kouros statue known as the Sounion Kouros and an impressive votive relief, both now in the Athens National Archaeological Museum. A column from the temple can be seen in the British Museum.