You can find a reference to Sounion in Homer's Odyssey. There, it is called as "holy Sunium, the headland of Athens". Headland because Athens didn't really stopped at the walls of its city but all of the province of Attica was considered as property of Athens. And holy, because the site of Sounion was dedicated to the god Poseidon.
I visited this place in 2010. It was actually the first place during my stay in Greece that I visited, outside of Athens, and it was well worth the bus trip.
It was inspiring and humbling to think that more than 2000 years ago, the Athenians considered this point as the frontier of their homeland. How sad must have been their thoughts when they saw this promontory recede on the horizon. And how uplifting must it have been to see it arise, months later, when they were coming back!
Thanks to its history and its romantic charge, the ruins have been visited a lot of time, especially during the XIXthe century. It is said that among the columns, you can find a graffiti made by Lord Byron himself, at the time when such thing was common. It was the kind of Instagram of the day back then! To be frank, I could not see it, since to approach the columns was restricted.
The funny thing also was to find partridges nest and prosper in the bushes around the temple:
Photo credits: all mine!