Sounio (or Cavo kolones or Cavokolones) is the name of the cape located at the southernmost tip of the prefecture of Attica. Its coasts are rocky and steep. It rises almost vertically from the sea to a great height, forming at the foot of it to the west a small cove which is separated by an isthmus from another eastern cove. Sounio is known because of its important geographical position but also because of the ruins of the ancient temple of Poseidon found there. In recent years, the wider area, due to its environmental value, was declared a national forest. It is named after the sunset from Sounio.

The Temple of Poseidon was built in 444–440 BC. during the period of the politician Pericles, who also rebuilt the Parthenon in Athens. It was built on the ruins of a temple dating back to the Archaic period. It is located above the sea at a height of almost 60 meters (200 feet). The design of the temple is a typical hexastyle, that is, it had a front portico with six columns. Only a few columns of the Temple of Sounio survive today, but when intact it would have closely resembled the modern and well-preserved Temple of Hephaestus below the Acropolis, which may have been designed by the same architect.