An ancient city at the mouth of a mighty river was the recipient of untold wealth due to the shipping trade, but changing conditions along the coastline left it sunken under water. This is the story of the lost Egyptian city of Thonis, also called Heracleion. Once thought to be a myth, it was found exactly where it always was, except underwater in the Mediterranean Sea. The stories of Heracleion involve both real people (Cleopatra) and mythological characters (Helen of Troy), but no trace of the city was seen for a thousand years. Heracleion's ruins are filled with expensive artifacts, leading experts to believe the disappearance of the city was relatively sudden, or else those treasures would have been moved. The upshot is that you can build flimsy structures or sturdy stone structures, but if your foundation is on the sandy coastline, you're at the mercy of the sea. New Orleans, are you taking notes? -via Digg
PS: Don't let the video title "Filthy Secrets" deter you from watching. There's nothing filthy in the video.