Surveying the Sea of Galilee in Israel in 2003 with sonar, a group of researchers found a “cone-shaped” pile of unhewn basalt boulders — and they have no idea exactly what it is.

Large Man Made Structure Found Under Water in Sea of Galilee

Sonar spotted the structure first in 2003. (Image: Shmuel Marco via International Journal of Nautical Archaeology)

Publishing of their findings in the latest issue of the International Journal of Nautical Archaeology, the researchers from several Israeli universities describe the “submerged monumental structure” as not resembling any natural feature, leading them to believe it is man-made. If this is the case, the boulders composing the entire structure — 70 meters (about 229 feet) total in diameter — would have to have been moved and the nearest natural outcrop of similar rock is a few hundred meters away.

Here’s how the paper describes the pile:

The highest is a circular stone pile,c.7 m in diameter and 2 m high. It is located at the saddle connecting the topographic ‘peninsula’ to the beach. Its base has an elevation ofc.-212 m. The second, a rectangular, elongated stone structure measuring about 20×10 m, lies in an axis parallel to the beach and almost 100 m to the east of the round structure. It has a base elevation of c.-214 m and stands about 0.6–0.8 m in height. To the south of these there was a disturbed semi-circle of large basalt stones at c.-213 m, which may be the remains of a small quay. None of these have been excavated, thus it can only tentatively be suggested that they were constructed during the Hellenistic/Roman/Byzantine period, through similarities with other remains found around the lake

Large Man Made Structure Found Under Water in Sea of Galilee

An illustration showing a reconstruction of what the structure under the sea looks like. (Image: Shmuel Marco via International Journal of Nautical Archaeology)

What could such a structure be used for? The researchers speculate that it could have attracted fish and might have been used as a fish nursery. Similar nurseries in the Sea of Galilee were considerably smaller than this structure though, the research notes.

If the structure was built out of the sea, the researchers write, it could have later become submerged under water due to natural occurrences like movement of tectonic plates relatively close by or rising of sea levels due to change in climate.

Large Man Made Structure Found Under Water in Sea of Galilee

A photo of the boulders taken by scuba divers. Basalt boulders were measured to be up to 1 meter long. (Photo: Shmuel Marco via International Journal of Nautical Archaeology)

Given the sheer estimated weight of the structure — 60,000 tons — the researchers inference the society that built it up to 4,000 years ago was “well organized” and with “economic ability.”

“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,” Yitzhak Paz with the Israel Antiquities Authority and Ben-Gurion University told LiveScience.

If this dating is correct, a nearby city at the time would have been ”Bet Yerah” or “Khirbet Kerak,” which LiveScience reported Paz saying was a “powerful and fortified town” in the region and in all of Israel.

Further research needed to learn more about the site includes underwater evaluation of its foundation and a search for artifacts, as well as dating to provide a more concrete time of when it could have been built.

Read more details about the structure in the Sea of Galilee in the journal article here.

(H/T: Yahoo!)