Secret tunnels discovered under a castle thanks to Leonardo da Vinci's manuscript

Modern surveying has confirmed that long suspected underground passages run beneath Milan’s Sforza Castle, thanks to the drawings in one of Leonardo da Vinci’s notebooks.

A 2025 investigation by the Politecnico di Milano and led by researcher Francesca Biolo, used ground penetrating radar and laser scanning to verify structures that align with

That notebook is specifically Codex Forster I, a small volume that includes pages Leonardo wrote in mirror writing during the late 1400s. It includes hydraulic sketches and notes on measuring solids.

Leonardo da Vinci and Sforza Castle

Leonardo spent the 1490s at the Sforza court, painting rooms and advising on projects at Sforza Castle. He annotated practical ideas, and he drew parts of the site with an eye for how people and materials could move.

The notebook is not a polished treatise. It is a working record, a place where engineering, art, and logistics sit side by side.

The pages that refer to subterranean features match the period when Ludovico Sforza pushed building work in Milan. The timing matters because it explains why Leonardo captured details that ordinary visitors never saw.

Sforza Castle tunnels

The team turned to ground penetrating radar to send radio waves into the soil and measure reflections, which can flag voids and foundations.

Signals revealed features about 8 to 12 inches below the surface, which allowed precise mapping without excavation.

They paired that with laser scanning, a method that sweeps structures with light pulses to collect millions of points and assemble an accurate 3D model.

The result is a framework that can be updated as new evidence appears.

Those data feed a digital twin, a georeferenced model used to test scenarios, plan maintenance, and coordinate research.

The work sits within geomatics, the discipline that measures and analyzes the built and natural environment.

What the scans revealed

The radar and scans identified previously undocumented cavities and walkways around a defensive perimeter once called the Garland.

Some of these lie near known but inaccessible rooms, which helps explain how internal circulation might have worked when the fortress served military needs.

Researchers note alignments between the subsurface features and Leonardo’s pages. That correspondence gives weight to the view that his sketches were based on direct study of the site rather than hearsay.

Historical accounts speak of an underground route tied to Ludovico Sforza’s family life. A recent report notes a corridor toward the Basilica of Santa Maria delle Grazie, where Beatrice d’Este was laid to rest.

Digital twin of Sforza Castle

A 2023 paper by the Politecnico team shows how a digital twin supports constraint maps that mark caution zones for future work.

That approach reduces the risk of accidental damage by steering heavy equipment and utilities away from sensitive volumes.

It lets researchers combine instrument readings with archival sources when structures have vanished. When the model flags a likely tunnel, historians can check drawings and records to see if the geometry and depth match.

The system is also a learning tool for curators and the public. There are plans to layer augmented reality on virtual paths so visitors can explore spaces that remain off limits.

What changes for access and conservation

Newly mapped routes will not open tomorrow, which is a good thing. The goal is to stabilize fragile areas, evaluate airflow and moisture, and make sure any access is safe for people and for the structures themselves.

Conservation teams can now target questions that were impossible a year ago. They can ask whether a void is a drain, a sally port, or a later excavation by utilities, and they can answer without taking a shovel to the ground.

The work reframes how we read the city above. Streets, courtyards, and galleries sit over layers of history that can be studied responsibly with the right tools and shared in ways that do not put them at risk.

Voices from the project

“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 Biolo. 

The project aims to build a digital twin of Sforza Castle, a virtual reconstruction that captures not only the present-day structure but also integrates evidence of lost historical features, offering a way to study elements no longer visible. 

Forster I shows that careful observation pays off across centuries. Its pages do more than illustrate ideas, they help solve real questions about a living monument.

The discovery rests on a simple chain of steps. A historical source points to a feature, a modern instrument checks it, and a shared model keeps the evidence clear.

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