Here’s what you’ll learn when you read this story:An underground tunnel network long rumored thanks to drawings by Leonardo da Vinci under Milan’s Sforza Castle are proven to exist. Experts plan to create a digital twin of the castle’s underground system for museum visitors. And recently, a team of researchers discovered one of those surprises—one drawing of a castle tunnel from a 1400s-era castle. See, the drawing was of Sforza Castle, and it included depictions of numerous underground tunnels—tunnels that had never been found. He turned it into the Sforza Castle, complete with towers, courtyards, and frescoed-lined halls.