Sounion is on the southern tip of Attica region and the temple of Poseidon is one of the most significant ancient monuments in the country. My friend, Jerry, (who says he´s fed up with the animals in ADP) went there and sent me two pics.
Legend has it that Aegeas, King of Athens and father os Theseus, dived to his death from this spot into the sea when he thought, wrongfuly, that his son was dead after a fight with Minotaur in Crete. The sea was named after him, Aegean Sea.
From here, you can see most of the Aegean islands that lie near Attica. The English poet Lord Byron carved his name onto one of the columns.
Sounion at Wikipedia
Sounion at Google Maps (the temple is just outside the hi-res area)