Leonardo da Vinci’s art has been pored over for centuries — The Last Supper, Vitruvian Man, Mona Lisa, and so much more. Still, his work can surprise us. A research team has just revealed one of those surprises: a long-overlooked sketch that appears to map underground tunnels beneath a 15th-century Milan fortress.
The drawing focuses on Sforza Castle and shows a web of passageways running below the complex — routes that, until now, had never been physically confirmed.
To investigate, specialists from Polytechnic University of Milan, Codevintec, and Sforza Castel combined ground-penetrating radar with laser-scanning to create a detailed model of the castle’s subterranean layers. Their scans didn’t just support Leonardo’s notes; they uncovered evidence that the tunnels he sketched are real and may belong to a much larger, interconnected system stretching underneath the site.
“The ground-penetrating radar enriched the 3D model with data on known, but inaccessible, spaces, bringing to light unknown walkways and ideas for further studies on secret passages,” said Francesca Biolo of Polytechnic University of Milan.
Sforza Castle was transformed in the 1400s when Duke Francesco Sforza rebuilt the earlier stronghold into a grand residence with defensive towers, open courtyards, and frescoed interiors. Decades later, his successor Ludovico Sforza expanded the project and brought Leonardo into the court. According to Art Net, while working for the Sforza family, Leonardo documented the castle’s hidden tunnel system in Codex Forster I.
Historians think these passages served practical military functions, but some likely had more personal purposes. One corridor appears to link the castle to the Basilica of Santa Maria delle Grazie — the church that houses Leonardo’s The Last Supper and serves as a burial place for the Sforza family. Such a route could have offered a discreet, protected way for the family to reach private tombs, including that of Ludovico’s wife, Beatrice d’Este.
Today, the castle hosts three museums: Pinacoteca del Castello Sforzesco, the Museum of the Rondanini Pieta, and the Museum of Ancient Art. While much of the underground network remains closed to the public, the same technology used to verify its presence may soon make it easier to explore — at least virtually.
“The goal is to create a digital twin of the Sforza Castle, a digital model that not only represents the current appearances of the castle but also allows you to explore the past, recovering historical elements that are no longer visible,” said Franco Guzzetti, professor of geomatics at Polytechnic University of Milan.
Researchers hope that augmented reality layered onto these digital routes could let visitors “walk” through sealed spaces and glimpse lost architectural details. In a sense, Leonardo’s curiosity is still opening doors — or in this case, tunnels — more than 500 years later.
