The oldest we've found, but if they were building something this elaborate 11,000 years ago, there have to have been simpler sites even earlier, even if they have not survived. (I'm thinking of the "monolith to El" in Michner's The Source.)
12,000 years ago the Arabian/Persian Gulf was dry land. Up until 7,500 years ago the present day Black Sea was called the old Euxine Lake (the ancients referred to what we call the Black Sea as the new Euxine Sea after the isthmus collapsed and sea water flooded into the old lake’s basin).
The shoreline of India at one time extended out 33 kilometers further than it does today. There are flooded cities offshore, some with pyramids.
Marine archeology in India is progressing nicely though you will never hear about it from western archeologists. There is passionate disagreement between European and Indian archeology over how long humans have had established civilisations. The Indians insist it goes back hundreds of thousands of years.
