Beneath Sforza Castle in Milan, researchers have discovered a series of secret tunnels—passageways once sketched by the Renaissance artist and scientist Leonardo da Vinci and traversed by a widower sick with grief.

The tunnels were discovered by experts from the Polytechnic University of Milan, who used ground-penetrating radar and laser scanners to map the 15th-century stronghold’s subterranean structures. The imaging revealed cavities and passageways positioned just a foot or two below the ground.