The Fosse Dionne is a mysterious spring at the heart of Tonnerre in France, with an unusually high flow of water. Since medieval times, myths and legends have evolved around the source of this spring, and divers who have tried to uncover its origins have lost their lives. Now a new diver has been permitted to try and go further than anyone has gone before, but will they finally get answers to the questions that have surrounded this spring for hundreds of years? Video by Mathieu Orcel and Agustin Muniz