(I have edited the second half of your query since only one question is allowed per eNotes post.)
The lost city of Atlantis was once believed to be only a myth, first mentioned in writings by Plato. Believed to be in existence around 9600 B. C., it was positioned "in front of the Pillars of Hercules," before catastophically submerging beneath the sea in just one day. Theorists believe Atlantis may have been an island in the Mediterranean, ranging anywhere from the larger islands (Crete, Cyprus, Malta) to Turkey (near Troy or in the Black Sea), the Middle East (Israel) and even North Africa and the Straits of Gibraltar (Andalusia). A large tsunami, the Thera eruption, is known to have disrupted other civilizations in the area (Crete) around 1700 B. C., and some people believe Plato's time placement was merely an error in translation. Other locales in the Atlantic Ocean (Ireland, the Canary Islands) have been suggested, though there is no scientific evidence (via geological studies on the ocean bottom) to support this. Even the area of the Bermuda Triangle has been proposed, along with Cuba and The Bahamas. Remains of human habitation have been found in many underwater locales, particularly in the Black Sea (which also suggested that it could have been the remains of Noah's Ark). Speculation will continue concerning this popular myth, but it is doubtful that the location of the lost city of Atlantis will ever be unequivocally proven.