A giant stone structure discovered beneath the waters of the Sea of Galilee - just south of the old city of Tiberias, in what is now Israel - was first detected in the summer of 2003 during a sonar survey of the southwest portion of the sea. It appears to be a giant cairn, with rocks piled on top of each other. It is conical, nearly 10 meters in height and about 70 meters in diameter - around 12,000 cubic metres.
      In the latest issue of the International Journal of Nautical Archaeology, researchers write: "Close inspection by scuba diving revealed that the structure is made of basalt boulders up to 1 metre long with no apparent construction pattern. The boulders have natural faces with no signs of cutting or chiselling. Similarly, we did not find any sign of arrangement or walls that delineate this structure." They say it is definitely human-made and probably was built on land, only later to be covered by the Sea of Galilee as the water level rose.
     Researcher Yitzhak Paz, of the Israel Antiquities Authority and Ben-Gurion University, believes it could date back more than 4000 years. "The more logical possibility is that it belongs to the third millennium BCE, because there are other megalithic phenomena [from that time] that are found close by," says Paz, noting that those sites are associated with fortified settlements. 
     Several examples of megalithic structures found close to the Sea of Galilee are more than 4000 years old. One is the site of Khirbet Beteiha, some 30 kilometres north-east of the submerged stone structure. It comprises three concentric stone circles, the largest of which is 56 metres in diameter.
     If the third-millennium BCE date idea proves correct, the underwater structure was about 1600 metres north of a city that researchers call Bet Yerah, or Khirbet Kerak - one of the biggest sites in the region at the time. Archaeologist Raphael Greenberg describes it as a heavily fortified 30 hectare site of up to 5000 inhabitants, with paved streets and towering defences.

Edited from Yahoo! News, LiveScience (9 April 2013)