A single fingerprint in ancient tar is rewriting the story of one of Scandinavia’s earliest seaborne raids.
Researchers have identified a fingerprint preserved in the tar used to seal the oldest known wooden plank boat in Scandinavia, offering a rare physical connection to the sea raiders who used the vessel more than 2,000 years ago. By closely examining the composition of the tar, scientists at Lund University are gaining new insight into the long-debated question of where these attackers originated.
During the 4th century BC, a small fleet of boats launched an attack on the island of Als off the coast of present day Denmark. The raiders, who may have traveled in as many as four vessels, were ultimately defeated. After the battle, the defenders placed their enemies’ weapons into the bog along with one of the boats, most likely as a ritual offering to mark their victory.
“Where these sea raiders might have come from, and why they attacked the island of Als has long been a mystery,” says Mikael Fauvelle, archaeologist at Lund University.
A uniquely preserved war boat
The vessel was first discovered in the 1880s in the Hjortspring Mose bog, excavated more extensively in the 1920s, and later became known as the Hjortspring boat. It remains the only prehistoric plank-built boat ever found in Scandinavia. Because it was deliberately placed in a bog as an offering, the boat survived in remarkably good condition. It has since been displayed at the National Museum of Denmark.
When researchers recently located sections of the boat that had never undergone chemical preservation, they were able to analyze them using modern scientific techniques.
“The boat was waterproofed with pine pitch, which was surprising. This suggests the boat was built somewhere with abundant pine forests,” says Mikael Fauvelle.
Earlier theories proposed that the boat and its crew came from the area around present-day Hamburg in Germany. The new evidence instead points toward origins in the Baltic Sea region.
“If the boat came from the pine forest-rich coastal regions of the Baltic Sea, it means that the warriors who attacked the island of Als chose to launch a maritime raid over hundreds of kilometers of open sea,” says Mikael Fauvelle.
As for where the fingerprint itself was left, that question remains open. The most definitive way to determine the boat’s origin would be through tree year ring counting, which could link the wooden planks to the specific region where the trees were originally cut.
“We are also hoping to be able to extract ancient DNA from the caulking tar on the boat, which could give us more detailed information on the ancient people who used this boat,” concludes Mikael Fauvelle.
Detective work led to the discovery
The latest findings are the result of careful detective work by the researchers.
The team wanted to find material from the boat that had not yet been subjected to conservation. This involved going through the archive at the National Museum and reading old correspondence, detailing when and where materials had been shipped between different storage areas and museums in Denmark.
“When we located some of the boxes of materials, we were very excited to find that they contained samples from the original excavation that had not been studied in over 100 years,” says Mikael Fauvelle.
How the researchers examined their findings
The team used a wide range of modern scientific methods to study the Hjortspring material. They were able to carbon date some of the lime bast cordage used on the boat, giving them the first absolute date from the original excavation material and confirming its pre-Roman Iron Age dating.
They also used X-ray tomography to make high-resolution scans of the caulking and cordage material found on the boat. This included making a digital 3D model of the fingerprint found in some of the caulking tar.
They used gas chromatography and mass spectrometry to study the caulking material and to see how it was produced. In addition, they worked with modern rope makers to create replicas of the ship’s cordage to study the rope-making process used in the boat’s construction.
Reference: “New investigations of the Hjortspring boat: Dating and analysis of the cordage and caulking materials used in a pre-Roman iron age plank boat” by Mikael Fauvelle, Boel Bengtsson, Olof Pipping, Mikkel Hollmann, Martin Nordvig Mortensen, Peter Toft, Sahel Ganji, Ashely Green, Christian Horn, Stephen Hall, Flemming Kaul and Johan Ling, 10 December 2025, PLOS ONE.
DOI: 10.1371/journal.pone.0336965
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