A 127-kilo cocaine haul with a street value of almost €500,000 was found aboard the Spanish naval training ship Juan Sebastian Elcano docked in Cadiz.
The discovery was made by the Civil Guard who seized and searched the vessel when it returned to Spain following a six-month global voyage.
The drugs are thought to have been loaded in Cartagena de Indias, Colombia, where the boat stopped in mid-April.
In early May more than 20 kilos of cocaine loaded on ship was sold while it was docked in New York, with the Spanish authorities then tipped off by the US department of Homeland Security Investigations.
It took until last month when the ship moored off of the northwest Spanish city Pontevedra to detain three suspected sailors – two Spaniards and an Ecuadorian – for drug trafficking.
It is unknown whether the three had accomplices, though US authorities report the men were paid €3,800 for every kilo they trafficked.