A large wooden platform, dated to be older than Stonehenge, was found hidden beneath a man-made stone island in Scottish loch, the University of Southampton announced on Tuesday.

The artificial island, also known as a “crannog,” is located in Loch Bhorgastail on the Isle of Lewis.

“Crannogs are small artificial islands that are typically thousands of years old,” explained archaeologist Dr. Stephanie Blankshein of the University of Southampton. “Hundreds [of crannogs] exist in the lochs of Scotland and many remain unexplored or undiscovered.”

The study, which took place over several years, revealed the over 5,000-year-old layered timber construction tucked below the stone capping of the island, as well as hundreds of Neolithic pottery shards floating in the water nearby - indicating that the platform had been first built during this period.

“While we still don’t know exactly why these islands were built, the resources and labour required to construct them suggests, not only complex communities capable of such feats, but also the great significance of these sites,” Blankshein said. “Large quantities of pottery, often still containing food residue, and worked stone found on and around the islands, suggest their use for communal activities such as cooking or feasting.”

Using stereophotogrammetry, a technique that calculates the 3D shape and position of objects by analyzing the differences between photographs taken from different viewpoints, researchers were able to study the the crannog as a single continuous structure both above and below the waterline - a perspective not possible using only a land or underwater survey.

The analysis found that the crannog started out as a circular wooden platform, measuring at around 23 meters across, and topped with brushwood. During the Middle Bronze Age, about 2,000 years later, a second layer of brushwood and stone were added, with a third being layered atop some 1,000 years later during the Iron Age.

An ancient stone causeway leading from the loch’s shore to the crannog has also since been found, submerged in the waters of the loch.

Most notable about the fieldwork, was the archaeologists’ new technique for using stereophotogrammetry in shallow water in order to examine the loch bed surrounding the crannog, recently published in the journal Advances in Archaeological Practice.

Successfully using stereophotogrammetry in shallow water is not easy, noted Professor Fraser Sturt, director of the Southampton Marine and Maritime Institute.

“Fine sediments, choppy conditions, floating vegetation and distorted or reflected light all hinder shallow water imaging,” explained Sturt. “Photogrammetry is very effective in deep water but runs into problems at depths of less than a metre. This problem is a well- known frustration for archaeologists.”

In order to circumvent this, researchers used two small waterproof cameras with low light performance and a wide field of view in order to provide overlapping images to compensate for any missing or disrupted data.

A diver then carefully manoeuvred the cameras through the water, matching the pace and position of a drone launched above.

“By combining stereophotogrammetry, drone technology and some innovative post-processing of the data, we have managed to set out an accessible approach that is portable and cost effective,” Blankshein wrote.