An aerial view of the 600-year-old castle ruins in Vannes. Screengrab from Inrap's YouTube video Vannes is along the western coast of France and about 300 miles southwest of Paris.

About 10 feet down, archaeologists found the top of the castle ruins. The elaborate stone structure was built in the 1380s by the Duke of Brittany Jean IV to showcase his power, the institute said.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

An aerial view of the 600-year-old castle ruins in Vannes. Screengrab from Inrap's YouTube video Vannes is along the western coast of France and about 300 miles southwest of Paris.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.

Archaeologists in Vannes unearthed the ruins of an elaborate castle and moat built in the 1380s by a duke and later ... → Hidden tunnel network found at abandoned 800-year-old home in France.