Here’s what you’ll learn when you read this story:

• Researchers discovered a fingerprint on Scandinavia’s oldest plank boat, used by seaborne raiders over 2,000 years ago.
• The print was left in the caulking tar by one of the boat’s makers.
• Experts still hope the fingerprint can point to clues about who the raiders were and why they attacked the island of Als.

The mystery of why a group of seaborne raiders attacked the Danish island of Als over 2,000 years ago is one step closer to being solved. Researchers scoured Scandinavia’s oldest plank boat (likely used by the raiders), and discovered a fingerprint pressed into the tar caulking material. While this is an admittedly small piece of evidence, archaeologists still hope it can move them closer to figuring out just where the attackers came from.

The attack on the island of Als happened in the fourth century B.C.E., when an armada descended. Even with up to four boats in the attack, the island-dwellers were able to defend their home, sinking one of the boats in a bog (likely as an offering of thankfulness for victory) in the process. The boat—which was was first discovered in the 1880s and excavated in the 1920s—is now known as the Hjortspring boat, and is the only example of a prehistoric plank boat ever found in Scandinavia.

“Where these sea raiders might have come from, and why they attacked the island of Als has long been a mystery,” Mikael Fauvelle, an archaeologist from Lund University, said in a 2025 statement.

With the Hjortspring boat on display at the National Museum of Denmark, the university research team still unexpectedly found parts of the boat that hadn’t been chemically preserved, and studied them with modern tools. “The boat was waterproofed with pine pitch, which was surprising,” Fauvelle said. “This suggests the boat was built somewhere with abundant pine forests.”

Previously, scholars thought that the raiders must haveoriginated from modern-day Hamburg, Germany. But the researchers, who published their findings on the investigation into the lost pieces of the Hjortspring boat in the journal PLOS One, now say that the clues point to the Baltic Sea region as the origin point of the attack. The authors wrote that the use of pine pitch means the boats must have been made in a region abundant with pine forests, like the Baltic Sea region east of Rugen and Scania.

“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,” Fauvelle said.

The researchers used all the tools at their disposal to investigate the find, including carbon dating the lime bast cordage, which confirmed its pre-Roman Iron Age dating. They also used X-ray tomography for high-resolution scans, created a 3D model of the fingerprint, and employed chromatography and mass spectrometry to help see how the caulking tar was produced.

“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,” Fauvelle said.

Analyzing intact cordage fragments and imprints of cordage on caulking material helped experts divine the sewing and rope-making techniques originally used in the construction, and the partial human fingerprint provides a direct link to the ancient seafarers. “Together, these results shed new light on the methods and materials used to build Scandinavia’s first plank boats,” the authors wrote, “and raise new questions regarding our understating of early maritime societies in Northern Europe.”