The wrecks are mostly buried in mud, but the oldest is thought to be a mediaeval cog, dating from the 14th or 15th century.

Cog ships, widely used for trading in the Baltic Sea for centuries, had a single square-rigged sail and were usually built of oak. The vessel discovered is 23–25m long with a 7m beam. Protruding deck-beams with unusually high knees and a simple anchor-wheel indicate its age.

The other shipwreck is estimated to be from the 16th century and still stands upright, including its mast. Alongside the discovery of artefacts such as kitchen utensils and tools on board have been 20 barrels of osmond iron, a find said to be unprecedented.

Osmond iron was cast by a particular process developed in Sweden, and its export was banned after 1604.

“My pulse went up when I realised what we were looking at – I have never seen such well-preserved shipwrecks,“ said diver Jim Hansson, maritime archaeologist at the Swedish National Maritime Museums.

The Baltic Sea is able to preserve timber shipwrecks for centuries because its brackish water doesn’t support wood-boring organisms such as shipworms.

There are thought to be at least 100 intact ships on the seabed, according to Nina Eklöf, Project Manager of the Swedish National Maritime Museums’ Treasures of the Baltic Sea.

This project is set to culminate in the opening of a new maritime-archaeological museum next to Stockholm’s Vasa Museum in Stockholm in 2020.

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