The discovery of a wooden man-made “island” in Scotland dating to the Bronze Age is stumping UK archaeologists, who say it could be as old as Stonehenge, according to a new report.

The ancient structure found buried in Loch Bhorgastail on the isle of Lewis is a type of structure known as a crannog and was built thousands of years ago, although scientists have no clue as to how or exactly why such platforms were built in the first place, the BBC said.

“When we actually started excavating is when we realized that it was actually this coherent, quite large timber structure that was under what you would see as the stone island today,” University of Southampton archaeologist Stephanie Blankshein told the outlet.

“While we still don’t know exactly why these islands were built, the resources and labor required to construct them suggests not only complex communities capable of such feats, but also the great significance of these sites,” Blankshein said.

Hundreds of crannogs have been found in the region and are suspected to have possibly been used for community gatherings, including feasts and celebrations, suggesting advanced societies at the time.

Archaeologists studied the Bhogastail crannog using 3D imaging and discovered that it was not made of stone as initially believed. Rather, the stones rested on a layered wooden base and brushwood, with food residue and hundreds of fragments of Neolithic pottery on them, the BBC said.

The circular wooden platform is about 75 feet across and was added onto over the centuries, VICE said.

“A lot of work went into it,” Blankshein said. “We’ve had a lot of really good [construction] dates come out of it, and all the dates are aligning to about 3500 to 3300 B.C. across all the sites we’re seeing.”