The purpose of the tunnels remains shrouded in mystery to this day – most puzzling of all, each identical 100-foot-long (30.5m) tunnel leads to a brick wall, giving rise to a whole host of theories.

The origins of the subterranean passageways are no clearer. While a 2013 archaeological study dated the complex to before the Middle Ages, Walid Nazim, author of The Secret of Lyon's Fishbones, believes the tunnels could have links to the last Grand Master of the Knights Templar, William de Beaujeu.

According to Nazim, Beaujeu owned the land on which the Fishbones was built in the 13th century. The author thinks it's possible that the complex was used as a safe to store Templar treasures. Whatever the truth, this is one architectural mystery that may never be solved.

The world’s top-secret tunnels and hidden passages