Built in 1605 at the instructions of Henri IV, the Rogny locks allow the waters of the Briare canal to drop 34m from the Trézée Valley to the Loing Valley, part of a huge project to link the Mediterranean to the Atlantic and the Channel. 12 000 men worked on this feat of engineering under architect Hugues Cosnier. Opened in 1642, it remained in use until 1887. These days, six other locks keep the Briare’s water traffic moving.