Two shipwrecks discovered in shallow waters off the coast of Costa Rica in the 1970s have now been definitively identified as Danish slave ships from the early 18th century. The breakthrough came from advanced wood analysis and the discovery of distinctive yellow bricks.
Marine archaeologist Andreas Kallmeyer Bloch documents the underwater excavation of a shipwreck off the coast of Costa Rica. In the photo, the characteristic yellow bricks produced in Flensburg are visible. Also visible is the wood from the ship, which is over 300 years old.
The wrecks, located near Puerto Limón in Cahuita National Park, were long believed to be pirate ships. This assumption was only questioned after 2015, when American marine archaeologists found unusually coloured bricks at one of the sites. The bricks turned out to be Danish and unique for their time, prompting further investigation.
Excavation
In 2023, archaeological divers from the National Museum of Denmark and the Viking Ship Museum conducted excavations at one of the wreck sites, which lies four metres below the surface. They collected samples of timber, bricks and other items—including clay balls and pipes—for further analysis. The results have been recently published and confirm that the wrecks are the remains of the Danish slave ships Fridericus Quartus and Christianus Quintus.
Historical records
These ships departed Copenhagen in December 1708 en route to the Danish West Indies. On board were textiles, metal goods and weapons meant to be traded for enslaved people along the West African coast. The plan was to transport around 700 enslaved individuals. However, in 1710, both ships ran aground on Costa Rica’s reefs due to navigational errors. The Fridericus Quartus was reportedly set on fire, and the Christianus Quintus was wrecked after its anchor rope was cut in heavy surf.
Historical records suggest that many of the enslaved people escaped, though around 100 were recaptured and forced to work on cacao plantations in the region. The wreck sites have never been officially documented—until now.
Definitive findings
“The analyses are very convincing, and we no longer have any doubts that these are the wrecks of the two Danish slave ships,” said marine archaeologist David Gregory, research professor at the National Museum of Denmark and head of the new maritime research centre Njord. He led the underwater excavations with fellow marine archaeologist Andreas Kallmeyer Bloch.
“The bricks are Danish, and the same goes for the timbers, which are additionally charred and sooty from a fire. This fits perfectly with the historical accounts stating that one of the ships was burnt,” Gregory added.
Bloch described the excavation as “the craziest archaeological project” he has ever been involved in. He said it has been a long journey with many setbacks, but finding the location of such a dramatic part of Danish colonial history is incredibly important—not just for Denmark but for the local community in Costa Rica as well.
Timber and clay analysis
Tree-ring dating of the charred oak timber revealed that the tree had been felled between 1690 and 1695 in the western Baltic region, including parts of Denmark, northern Germany and southern Sweden. The bricks matched the so-called Flensburg yellow bricks, which were made from clay sourced from the Flensburg Fjord—distinctively Danish and not used elsewhere in Europe at the time.
Additional finds, including clay balls and Dutch-made clay pipes typical of early 1700s Danish ships, helped confirm the identification. Such pipes were rarely used for more than five years.
This was Costa Rica’s first official underwater archaeological excavation conducted in partnership with Danish institutions and Costa Rica’s National Museum and Archaeological Commission.
