(images via: university of nottingham)
Believed to have been submerged off the coast of Greece by a series of earthquakes around 1,000 BCE, Pavlopetri is the oldest-known underwater archaeological town site in the world. Unlike other underwater ruins, which are incomplete or difficult to verify as actual man-made structures, Pavlopetri has a complete town plan, including streets, architecture and tombs. It consists of about 15 structures, submerged about 10-13 feet underwater.
Discovered in 1967, the site has been routinely explored by the University of Cambridge and the University of Nottingham, the latter of which has an ongoing excavation project to find and date artifacts found on the ocean floor.