By: Owen Jarus, LiveScience Contributor
Published: 04/10/2013 07:17 AM EDT on LiveScience

A giant "monumental" stone structure discovered beneath the waters of the Sea of Galilee in Israel has archaeologists puzzled as to its purpose and even how long ago it was built.

The mysterious structure is cone shaped, made of "unhewn basalt cobbles and boulders," and weighs an estimated 60,000 tons the researchers said. That makes it heavier than most modern-day warships.

Rising nearly 32 feet (10 meters) high, it has a diameter of about 230 feet (70 meters). To put that in perspective, the outer stone circle of Stonehenge has a diameter just half that with its tallest stones not reaching that height.

It appears to be a giant cairn, rocks piled on top of each other. Structures like this are known from elsewhere in the world and are sometimes used to mark burials. Researchers do not know if the newly discovered structure was used for this purpose.

The structure was first detected in the summer of 2003 during a sonar survey of the southwest portion of the sea. Divers have since been down to investigate, they write in the latest issue of the International Journal of Nautical Archaeology. 

"Close inspection by scuba diving revealed that the structure is made of basalt boulders up to 1 m (3.2 feet) long with no apparent construction pattern," the researchers write in their journal article. "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." [See Photos of the Mysterious Sea of Galilee 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. "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," the researchers write.

More than 4,000 years old?

Underwater archaeological excavation is needed so scientists can find associated artifacts and determine the structure's date and purpose, the researchers said.

Researcher Yitzhak Paz, of the Israel Antiquities Authority and Ben-Gurion University, believes it could date back more than 4,000 years. "The more logical possibility is that it belongs to the third millennium B.C., because there are other megalithic phenomena [from that time] that are found close by," Paz told LiveScience in an interview, noting that those sites are associated with fortified settlements. 

The researchers list several examples of megalithic structures found close to the Sea of Galilee that are more than 4,000 years-old.

giant structure sea of galilee
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.

One example is the monumental site of Khirbet Beteiha, located some 19 miles (30 kilometers) north-east of the submerged stone structure, the researchers write. It "comprises three concentric stone circles, the largest of which is 56 m [184 feet] in diameter." [Gallery: Aerial Photos Reveal Mysterious Stone Structures]

An ancient city

If the third-millennium B.C. date idea proves correct it would put the structure about a mile to the north of a city that researchers call "Bet Yerah" or "Khirbet Kerak."

During the third millennium B.C. the city was one of the biggest sites in the region, Paz said. "It's the most powerful and fortified town in this region and, as a matter of fact, in the whole of Israel."

Archaeologist Raphael Greenberg describes it in a chapter of the book "Daily Life, Materiality, and Complexity in Early Urban Communities of the Southern Levant" (Eisenbrauns, 2011) as being a heavily fortified 74-acre (30 hectares) site with up to 5,000 inhabitants.

With paved streets and towering defenses its people were clearly well organized. "They also indicate the existence of some kind of municipal authority able to maintain public structures ..." Greenberg writes.

The research team says that, like the leaders of Bet Yerah, whoever built the newly discovered Sea of Galilee structure needed sophisticated organization and planning skills to construct it. The "effort invested in such an enterprise is indicative of a complex, well-organized society, with planning skills and economic ability," they write in their journal paper.

Paz added that "in order to build such a structure a lot of working hours were required" in an organized community effort.

Future exploration

Paz said that he hopes soon that an underwater archaeological expedition will set out to excavate the structure. They can search for artifacts and try to determine its date with certainty.

He said that the Israel Antiquities Authority has a research branch capable of excavating it. "We will try to do it in the near future, I hope, but it depends on a lot of factors."

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  • "Younger Brother Obsidian," as labeled on the north wall of the Maya city's house by an unknown hand, was painted in the 9th century A.D. Archaeologist William Saturno of Boston University excavates the house in the ruins of the Maya city of Xultún. Younger Brother Obsidian may have been the town scribe. Excavation and preservation of the site were supported by the National Geographic Society. Photo by Tyrone Turner © 2012 National Geographic

  • Trees grow atop a newly discovered mound over a house built by the ancient Maya that contains the rendering of an ancient figure, possibly the town scribe. The house sits at the edge of the ancient site of Xultún in Guatemala, a city that once housed tens of thousands of people. Excavation and preservation of the site were supported by the National Geographic Society. Photo by Tyrone Turner © 2012 National Geographic

  • Three male figures, seated and painted in black. The men, wearing only white loincloths and medallions around their necks and a head dress bearing another medallion and a single feather, were uncovered on the ruined house's west wall. The painting recreates the design and colors of the original Maya mural. Excavation and preservation of the site were supported by the National Geographic Society. Painting by Heather Hurst

  • A Maya king, seated and wearing an elaborate head dress of blue feathers, adorns the north wall of the ruined house discovered at the Maya site of Xultún. An attendant, at right, leans out from behind the king's head dress. The painting by artist Heather Hurst recreates the design and colors of the original Maya artwork at the site. The excavation and preservation of the site were supported by the National Geographic Society. Painting by Heather Hurst

  • A vibrant orange figure, kneeling in front of the king on the ruined house's north wall, is labeled "Younger Brother Obsidian," a curious title seldom seen in Maya text. The man is holding a writing instrument, which may indicate he was a scribe. The painting recreates the design and colors of the figure in the original Maya mural. Excavation and preservation of the site were supported by the National Geographic Society. Painting by Heather Hurst

  • Four long numbers on the north wall of the ruined house relate to the Maya calendar and computations about the moon, sun and possibly Venus and Mars; the dates may stretch some 7,000 years into the future. These are the first calculations Maya archaeologists have found that seem to tabulate all of these cycles in this way. Although they all involve common multiples of key calendrical and astronomical cycles, the exact significance of these particular spans of time is not known. Illustration by William Saturno and David Stuart © 2012 National Geographic

  • The painted figure of a man -- possibly a scribe who once lived in the house built by the ancient Maya -- is illuminated through a doorway to the dwelling, in northeastern Guatemala. The structure represents the first Maya house found to contain artwork on its walls. The research is supported by the National Geographic Society. Photo by Tyrone Turner © 2012 National Geographic

  • Conservator Angelyn Bass cleans and stabilizes the surface of a wall of a Maya house that dates to the 9th century A.D. The figure of a man who may have been the town scribe appears on the wall to her left. Excavation and preservation of the site were supported by the National Geographic Society. Photo by Tyrone Turner © 2012 National Geographic

  • Never-before-seen artwork -- the first to be found on walls of a Maya house -- adorn the dwelling in the ruined city of Xultún. The figure at left is one of three men on the house's west wall who are painted in black and wear identical costumes. Excavation and preservation of the site were supported by the National Geographic Society. Photo by Tyrone Turner © 2012 National Geographic

  • Archaeologist William Saturno of Boston University carefully uncovers art and writings left by the Maya some 1,200 years ago. The art and other symbols on the walls may have been records kept by a scribe, Saturno theorizes. Saturno's excavation and documentation of the house were supported by the National Geographic Society. Photo by Tyrone Turner © 2012 National Geographic