The craft was spotted off Cap Bon by managers of a diving club in the country’s northeast as they were exploring new sites student divers.
The almost intact wreck is covered in algae, its hatches and periscope now home to fish and crustaceans.
After asking several experts, the club concluded it could only be the Ariane, which was based in Bizerte, at the time a French port in northern Tunisia.
This is the third submarine found in Tunisia, and the only one from the First World War. It’s exciting, as if a history book has come to life,” said Baccar.
During the First World War, German submarines wreaked havoc off the Tunisian coast, where they were initially deployed to cut the Allies off from their reinforcements of men and provisions from the French colonies, said historian Ali Ait Mihoub, from Manouba university.
About 80,000 Tunisians were mobilised to fight or work in French factories during WW1.