The Medieval Ship Under Tallinn That Still Had Its Compass

On the morning of March 31, 2022, an excavator operator digging foundations for an office building at Lootsi Street 8 in Tallinn hit something solid in the wet sand. Archaeological monitoring stopped the work within hours. What came out of the ground over the following weeks was not a few scattered timbers from a ruined medieval vessel but something considerably rarer: a large, largely intact 14th-century merchant ship, preserved by the damp marine sediment that had sealed it off from the world above for more than six hundred years. And inside it, among worn leather shoes, tools and the preserved remains of two ship rats found in spilled tar, was a dry compass. If the identification holds, it is the oldest surviving example of its kind in Europe.

Now, a new dendrochronological study published in Dendrochronologia by Aoife Daly and colleagues from Dendro.dk Copenhagen and the University of Tartu has gone deep into the ship’s timber, establishing not just when its trees were felled but where they grew and what secrets their rings carry about the forests, trade routes and shipbuilding practices of the late medieval Baltic world. The picture that emerges is more precise, and more surprising, than anyone expected when the excavator first broke ground.

A Ship Sealed Beneath a Changed World

The area around Lootsi Street looks nothing today like the landscape the ship knew. In the 14th century, the vessel rested near shallow coastal waters close to the delta of the Härjapea River, a few hundred metres from the medieval harbour of Tallinn, then one of the most commercially active ports on the Baltic Sea. The marshy ground was reclaimed from the sea in the early 19th century, raised with landfill, and built over with warehouses, a railway and eventually a car park. The ship lay beneath all of it, perfectly undisturbed.

The vessel, now known as the Lootsi 8 cog, measures 24.5 metres long and 8.6 metres wide. Its preservation is exceptional: archaeologists describe it as one of the best-preserved medieval ships found anywhere in Europe. It was so large and structurally fragile that specialists had to saw the wreck into four bulk sections and transport it to the Estonian Maritime Museum, where a dedicated tent structure was built to allow full excavation and study away from the construction site. That unusual circumstance turned out to be an extraordinary scientific opportunity.

The ship belongs to the cog tradition, a broad medieval vessel type associated with cargo transport across the North Sea and Baltic. Cogs are closely linked to Hanseatic trade, the network of northern European merchant cities that dominated commerce across the region from the 13th century onward. With its felling dates in the early 1370s, Lootsi 8 sits among the later iterations of the type, built just a few years before the famous Bremen Cog and within two decades of the last known examples of the ship type in the archaeological record. The disorder of finds aboard, scattered tools, personal objects, worn and repaired shoes left behind, points toward a sudden abandonment rather than a controlled discard. Why a ship this size was lost within sight of the port of Tallinn remains an open question.

What the Tree Rings Revealed

The dendrochronological study analysed 97 wood samples collected across three seasons in 2022, 2023 and 2024. Eighty-seven were successfully dated. The precision of the results is remarkable. Trees for the main hull structure were felled in the winters of AD 1370 to 1371 and again in 1371 to 1372. Planks for a possible galley or pantry structure at the stern, along with loose timbers interpreted as cargo, were made from trees felled in winter 1373 to 1374. The spread of felling dates maps the construction timeline: the lower hull took roughly two winters of timber procurement to complete, and the internal fitting out continued for at least another two years.

Crucially, the hull shows almost no evidence of major repair. Only minor caulked cracks were recorded on the outer planking. For a vessel built of high-quality oak with exceptionally few knots and twists in the grain, the absence of significant repairs suggests this was a relatively young ship when it sank. The combination of a new vessel and a suddenly abandoned crew makes the wreck feel less like a gradual decommissioning and more like an interrupted voyage.

Once all 87 samples were dated, the team compared the ring patterns of each timber against broad networks of oak tree-ring datasets from across northern Europe, a process called dendroprovenance. The results traced the ship’s timber back to its forests of origin with striking specificity. The two main groups of timbers, representing the majority of the hull planks and frames, correlate most strongly with oak chronologies from the coastal region of Lithuania. A smaller third group shows similar Baltic origins. A fourth small group matches a different source, probably slightly further north or inland, possibly toward the Riga region. Only one small group of cargo timbers points to a local Estonian source, correlating with oak from the city walls of Tallinn itself.

That last detail is one of the most suggestive findings in the entire study. The cargo timbers, felled in the same winter as the galley planks (1373 to 1374), came from a source matching a door still in use in the Bremen Tower of Tallinn’s medieval city walls. The researchers raise the possibility that the cog’s lower hull was built in coastal Lithuania, then sailed to Tallinn, where the galley was fitted out with locally sourced planks and the cargo was loaded for its first voyage. The ship never completed it.

Baltic Forests in the Ship’s Bones

The Lithuanian timber connection reaches further than the 1370s. One of the strongest correlations in the study links the main hull timbers of Lootsi 8 to planks from an entirely different ship, the Varberg 1 wreck found in southwest Sweden, whose planks were felled exactly one century later in winter 1471 to 1472. The same coastal Lithuanian forest that supplied oak for a Tallinn cog in the early 1370s was still supplying oak for shipbuilding in Sweden a hundred years on. Medieval timber networks were not opportunistic or ephemeral. They were stable, deep-rooted commercial relationships that persisted across generations, linking specific forest regions to specific markets across the entire Baltic world.

This is one of the quieter but more significant contributions of the Lootsi 8 study. Dendroprovenance turns a ship into a map of the trade system that built it. The timber in the hull, the planks on the keel, the frames and beams, all carry the chemical and structural signatures of the forests they came from and, by implication, of the merchants, woodcutters and transporters who moved them from forest to shipyard. The Lootsi cog was not just moving goods across the Baltic. It was itself an assembled product of the same networks.

Damaged Wood and the Shipbuilders Who Knew It

The study’s most unexpected discovery concerns the condition of the timber itself. Twenty-one of the 87 dated samples, roughly 24 percent, displayed what dendrochronologists call moon rings: pale bands of included sapwood that form when a growing tree is damaged, whether by severe frost, by the loss of a major branch, or by a combination of both. In studies of German oaks, moon rings appear in roughly one percent of trees. In the Lootsi 8 ship, the proportion is more than twenty times higher, a striking anomaly that the authors examine carefully.

The moon rings in the Lootsi 8 timbers cluster around three periods: approximately AD 1175, 1219 and 1330. Their synchronous occurrence across multiple trees in the same provenance group suggests that frost was the primary cause, a cold event severe enough to damage many trees growing in the same forest at the same time. The 1330 phase is particularly well documented: one cross-section shows an abrupt boundary between the last pale moon ring, formed in that year, and the first fully developed heartwood ring, formed the following year, exactly the kind of rapid transition consistent with a damaging frost event.

What is most revealing is not the damage itself but the response to it. In areas of the hull planking where moon rings were present, the shipbuilders placed long reinforcing planks, called battens, on the inboard side of the hull. They could see the pale anomalous rings in the cross-sections of the timber, and they understood that wood with moon rings carried structural risks. Rather than rejecting the compromised material, they used it anyway and compensated for its weakness with additional support. It is a small detail, but it brings the intelligence of medieval shipbuilding into sharp focus: these were craftspeople who read the anatomy of wood with practiced eyes and built defensively against what they found.

Beneath the modern streets of Tallinn, a ship rat population, a compass that may have no surviving equal, and the bones of a Lithuanian forest pressed into service for a last voyage are waiting, still, for everything the Lootsi 8 cog has yet to give up. A second, older wreck has been confirmed nearby, deliberately left in the ground until the tools exist to recover it properly. Tallinn is still building upward. Its medieval harbour is still pushing back.

Sources

• Daly, A., Sohar, K., Läänelaid, A., Lätti, P., & Reinvars, L. Timber for a medieval cog: Wood studies of the Lootsi 8 wreck, Tallinn. Dendrochronologia, 97, Article 126519 (2026)
• One of Europe’s Best-Preserved Medieval Ships Was Found Beneath a City, With Its 600-Year-Old Compass (Arkeonews)
• Wreck of the Lootsi Cog (Estonian Maritime Museum)