Mysterious tunnels sketched by Leonardo da Vinci in 1495 may finally have been discovered — hidden under a castle in Milan

Mysterious underground passages sketched by Leonardo da Vinci have finally been identified below Sforza Castle in Milan. The experts who found the tunnels, which are part of early military defenses, suggest that there may be even more structures waiting to be discovered.

Construction on Sforza Castle, which stands in the heart of modern-day Milan, began in the mid-1300s. Over the centuries, it was expanded, modified and partly demolished, and today, only about one-sixth of the original castle remains. Around 1495, the lord of Milan hired Leonardo da Vinci and other artists to decorate the castle's interior walls and ceilings. During this time, da Vinci sketched defensive fortifications that bear a striking resemblance to Sforza Castle, including multiple passageways that had been lost to time, until now.

"There are still underground passages, some of which are open for public visits," Francesca Biolo, a research fellow in architecture at the Polytechnic University of Milan, told Live Science in an email, but "it became clear that there might be much more to these pathways than what is currently known and accessible."

As part of her dissertation research project, Biolo collaborated with the Sforza Castle authorities and the engineering company Codevintec Italiana to develop and test a new method of digitizing complex, highly stratified historical buildings that uses multiple techniques.

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Biolo and her team conducted a survey of the underground spaces at the castle. To do so, they employed several techniques, including laser scanning, GPS, 3D ground-penetrating radar surveys and photogrammetry, which uses many photographs to create a virtual 3D model. Their objective was to determine whether there were spaces or passageways under the castle that were inaccessible and previously unknown.

"The results were far more significant and intriguing than we had anticipated," Biolo said. "We uncovered rooms on a second underground level and an additional passage running parallel to the known one."

These newly discovered passageways are likely among those illustrated in da Vinci's 1495 sketch.

"Leonardo da Vinci was highly knowledgeable about military structures and defensive systems of his time," Biolo said. In his sketch, the main body of the castle is clearly recognizable, along with multiple passageways. "That said, it is not possible to determine the exact extent of Leonardo's involvement in the structures of the Milanese fortress," she said.

Biolo added that there is likely much more to be discovered under Sforza Castle but that excavation is not an option.

"We would all be keen to see and physically explore these spaces, but it is also essential to respect the current state of the area and the city's constraints," she said.