Archaeologists have learned more about an ancient, monumental stone structure discovered in the Sea of Galilee.

The structure is a large collection of basalt rock and boulders and researchers say the pile of rocks resembles a burial marker, but they are not certain of its true purpose.

At 230 feet (70 meter) in diameter, the edifice is twice the diameter of Stonehenge, where the tallest stones do not reach the monument's 32-foot (10 meter) height. Many basalt boulders up to one meter in length were used to create structure, which researchers are calling a cairn.

"The shape and composition of the submerged structure does not resemble any natural feature. We therefore conclude that it is a man-made and might be termed a cairn," researchers said, according to Australian news site news.au.com.

The structure was first spotted in 2003 during a sonar scan of the Sea of Galilee. Researchers have since taken a closer look by scuba diving to the cairn, where they found it to have no distinguishable walls, divisions or construction patterns. Pictures of the structure appear make it look like a massive mountain of rocks.

One estimate puts the structure at 4,000 years old, which is similar to the age of other nearby ancient structures.

"The only period in this region for which megalithic structures can be connected to settlement sites is the Early Bronze Age, between the late 4th and the late 3rd millennia BCE," researchers said in the study.

To date, no underwater excavation of the structure has been carried out.

The Sea of Galilee is the largest freshwater lake in Israel, and at nearly 700 feet (200 meters) below sea level, it Earth's lowest known freshwater lake. 

See photos of the site and drawings published in the paper here