In the 4th century BCE, at least four wooden plank boats sailed across the island of Als off the coast of Denmark. The unknown attackers were defeated, the defenders driving their enemies’ weapons into the bog aboard one of these boats, known as the Hjortspring boat, most likely to thank them for their victory. Archaeologists have now carbon dated and analyzed the caulking and rope materials found with the boat and have also discovered a partial human fingerprint found on a fragment of caulking material. This remarkable fingerprint establishes a direct link with the ancient sailors who used the Hjortspring ship.
Photo of a caulk fragment showing the fingerprint on the left and a high-resolution X-ray tomogram of the fingerprint region on the right. Image credit: Erik Johansson / Sahel Ganji.
The Hjortspring boat was discovered in the 1880s during peat excavations in the Hjortspring bog on the island of Als, southeast Jutland, Denmark.
Formal investigations of the bog and the discovery of a boat began in 1920 after the reunification of southern Jutland with Denmark.
The boat was excavated by Gustav Rosenberg between 1921 and 1922 and approximately 40% of the original boat was recovered from the bog, allowing a complete reconstruction of the boat’s form.
Enough iron spikes and shields to equip a military band of about 80 warriors were dropped off with the boat – far more individuals than could have fit inside the boat itself.
Based on these findings, it has been suggested that a group from an unknown location traveling in four Hjortspring-style boats attacked the island of Als and were defeated, with the victors laying down the weapons of their defeated enemies along with one of their boats in the bog to give thanks for the victory.
After the excavations, the boat was preserved and has been on display at the National Museum of Denmark since 1937.
In the new study, Lund University archaeologist Mikael Fauvelle and colleagues carbon-dated and analyzed some previously unstudied caulking and cording materials found with the boat.
Using gas chromatography and mass spectrometry, they determined that the caulk was likely composed of animal fat and pine pitch.
At this time in its history, Denmark itself had few pine forests.
“It is possible that pine pitch reached Denmark via trade,” the researchers said.
“However, other coastal areas along the Baltic Sea to the east of Denmark had pine forests, leading us to believe that the ship may have been built in these areas and that he and his warriors may have come from that direction.”
“If this were true, the boat would likely have traveled a long distance on the high seas to reach Als, which could indicate a particularly organized and premeditated attack.”
The Hjortspring boat as it is currently on display at the National Museum of Denmark. Image credit: Boel Bengtsson.
Carbon dating of the ropes and caulking revealed that the boat was probably built sometime in the 4th or 3rd century BCE, which agrees with previous dating of the wood from the Hjortspring site.
Additionally, scientists discovered a partial human fingerprint in some of the caulking material during the study.
While they couldn’t determine exactly where the fingerprint came from, it could have been left by one of the crew members during a repair to the boat, providing a direct link to the sailors of the old ship.
“New analysis of Scandinavia’s oldest plank boat brings us closer to solving the 100-year-old mystery of the ancient boat’s origins,” they said.
“Using cutting-edge scientific methods, we identified the Baltic Sea region as the most likely source of this approximately 2,400-year-old ship, while discovering a fingerprint left by an ancient sailor in the tar used to waterproof the ship.”
“Finding a fingerprint on the tar fragments from the boat was a big surprise for us.”
“Fingerprints like this are extremely unusual for this period.”
“It’s great to have found a direct connection to one of the people who used this old boat.”
“The boat was used by a small army of invaders who attacked the island of Als in southern Denmark more than 2,000 years ago.”
“The invaders were defeated and the local defenders sank the boat in a bog as an offering to thank them for their victory.”
“Since the boat was dug out of the bog in the early 1920s, the question of where the invaders came from has remained an open mystery.”
“The weapons they used that were found in the boat were quite common for the time and were used throughout northern Europe, giving us little clue as to their origin.”
“Several different theories about the boat’s origins have been proposed over the past 100 years, with some researchers suggesting that the boat’s crew came from somewhere in northern Germany or perhaps another part of today’s Denmark.”
“Now our scientific analysis of the boat’s caulking material gives us the first major new clue in more than a century.”
“The boat was waterproofed with pitch from pine trees, which were rare in Denmark and northern Germany during the first millennium BCE.”
“We think this means that the boat and its crew most likely came from further east, along the shores of the Baltic Sea, where pine forests were more abundant.”
“The boat was excavated before modern dating methods were available and most of the boat’s materials were immediately preserved using chemicals that make radiocarbon dating impossible.”
“By going through the archives, however, we were able to find original ropes which had not been preserved.”
“We obtained a radiocarbon date from the rope that returned a date range between 381 and 161 BCE, confirming the pre-Roman Iron Age date of the boat.”
The study was published online in the journal PLoS ONE.
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Mr. Fauvelle and others. 2025. New investigations into the Hjortspring boat: dating and analysis of ropes and caulking materials used in a pre-Roman Iron Age plank boat. PLoS One 20 (12): e0336965; doi: 10.1371/journal.pone.0336965
