Leonardo da Vinci sketched an early diving-bell concept for underwater exploration.
A diving bell is an air-filled chamber lowered beneath the water’s surface. The trapped air provides a breathing space for divers, although the available air decreases as the bell descends and carbon dioxide accumulates. Leonardo’s studies explored ways a person might work underwater using a rigid air chamber.
The concept was not a modern submarine. A diving bell is generally open at the bottom and depends on trapped air, whereas a submarine is a self-contained vessel designed to travel underwater. Leonardo also made separate studies of underwater breathing equipment, but those should not be conflated with the diving bell.
His underwater designs were connected to Renaissance military and engineering interests, including harbor operations and possible attacks on ships. There is no reliable evidence that Leonardo’s complete diving-bell design was built and used operationally.