Scientists have discovered a giant structure, weighing 60,000 tonnes and twice the size of Stonehenge, under the Sea of Galilee in Israel.
The International Journal of Nautical Archaeology reports that the "monumental stone structure" was cone-shaped and made of rough basalt.
Researcher Yitzhak Paz, of Ben-Gurion University's Israel Antiquity Authority, said the structure was found in 2003 during a sonar survey and divers had now been down to investigate.
He told the journal that the structure could be 4000 years old.
"The more logical possibility is that it belongs to the third millennium BC, because there are other megalithic phenomena close by," Dr Oaz said.
At a height of 10 metres and with a diameter of 70 metres, the structure appears to be a giant cairn, used in many parts of the world to mark burials.
"Close inspection by scuba diving revealed that the structure is made of basalt boulders up to a metre long with no apparent construction pattern," researchers said. "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."
Putting all the data together researchers found that the structure is cone shaped, about 230 feet (70 meters) in diameter and nearly 32 feet (10 meters) tall. It weighs an estimated 60,000 tons.
The scientists say to monument was definitely manmade and probably built on land, only later to be covered by the Sea of Galilee as water levels rose.
"The shape and composition of the submerged structure does not resemble any natural feature. We therefore conclude that it is man-made and might be termed a cairn," they said.
The journal published a list of nearby examples of megalithic structures. One was the monumental site of Khirbet Beteiha, located 30km north-east of the submerged stone structure. It comprises three concentric stone circles, the biggest of which is 56 metres wide.
During the third millennium BC the city of Bet Yerah was one of the biggest sites in the region, Dr Paz said.
"It's the most powerful and fortified town in this region and, as a matter of fact, in the whole of Israel."
Dr Paz told the Journal of Nautical Archaeology that he was hopeful that an underwater archaeological expedition would soon excavate the structure.
The search will focus on finding artifacts and organic material in order to accurately date the site.