The tunnels they found are definitely plural though, only the connection to the sketches seems a bit tenuous. > It features a well-known underground passageway

The tunnels they found are definitely plural though, only the connection to the sketches seems a bit tenuous. > It features a well-known underground passageway that runs along the perimeter of the castle’s moat and is accessible to tourists. But to the researchers’ surprise, their survey revealed a second secret tunnel that experts had only hypothesized about for years. The second tunnel runs parallel to the first one, about 1 meter (3 feet) beneath the surface. [...] The researchers also uncovered other tunnels [...] including one that heads in the direction of the Basilica of Santa Maria delle Grazie, the resting place of the wife of Duke Ludovico Sforza.