Beneath Scottish waters, artificial islands have been hiding in plain sight for thousands of years.

A team of UK archaeologists has used a new imaging method to study submerged crannogs, artificial islands usually made of wood and stone, and confirmed that some are more than 5,000 years old, according to a recent paper in Advances in Archaeological Practice.

The findings push crannogs much further back than researchers had thought. They had long assumed the structures were mainly built, used and reused between the Iron Age and the post-medieval period.

The work comes from the larger Islands of Stone project, which has studied crannogs in the Outer Hebrides since around 2021. The region is a hotspot for the artificial islets, with 170 known sites out of 550 recorded in Scotland.

Researchers said crannogs have been hard to study because they sit partly on land and partly underwater. In the paper, they wrote: “Straddling the boundary between land and water, [crannogs] preserve complex stratigraphies that offer key insights into past occupations and changing hydrological landscapes.”

They also said standard land survey methods do not work in water, while marine geophysical equipment is not suitable in shallow water less than 1 metre deep. The paper describes this gap as a “white ribbon” of missing data.

To deal with that, the team used a modified form of photogrammetry, a method that creates 3D models by stitching together 2D images taken from different angles.

Fraser Sturt, a marine archaeologist at the University of Southampton and study co-author, said shallow water creates problems for imaging. “Fine sediments, choppy conditions, floating vegetation, and distorted or reflected light all hinder shallow water imaging,” he said.

“Photogrammetry is very effective in deep water but runs into problems at depths of less than a meter. This problem is a well-known frustration for archaeologists.”

The researchers used waterproof stereo cameras operated by divers, then compared that data with aerial drone surveys. They said the results matched well.

Using this stereophotogrammetry approach, the team filled in the missing data for a Neolithic crannog in Loch Bhorgastail in Scotland.

Radiocarbon dating showed the Loch Bhorgastail crannog is more than 5,000 years old. The team said it began as a circular wooden platform about 23 metres across, topped with brushwood.

About 2,000 years later, during the Middle Bronze Age, another layer of brushwood and stone was added. The island then preserved another phase of activity during the Iron Age, roughly 1,000 years later.

Hundreds of pieces of Neolithic pottery were found across the site, suggesting to the researchers that people first established it in that period.

Study lead author Stephanie Blankshein, an archaeologist at the University of Southampton, said: “While we still don’t know exactly why these islands were built, the resources and labor required to construct them suggest not only complex communities capable of such feats but also the great significance of these sites.”

According to Blankshein, some pottery with food residue suggests the islands could have been used for communal activities such as cooking or feasting.

For the final stage of the study, the team assessed how widely the method could be used and concluded it should give crannog investigations a big boost.

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