Send men to the bottom of the harbor in diving suits and let them cut holes in enemy hulls. Well, maybe that doesn’t sound so far-fetched any more. It’s fairly common now for frogmen with scuba gear to engage in underwater sabotage. In da Vinci’s time, however, the idea was unheard of. Da Vinci’s divers would have carried breathing hoses connected to a floating bell full of air, wearing facemasks with glass goggles that would help them see underwater.