Leonardo worked continuously and with originality on his study of water, and many drawings and observations are to be found scattered throughout the various notebooks. The idea of drawing all this long experience together in a treatise occurred to him several times, but the attempt came to an end almost as soon as it was begun. His interest in water probably began when he was a boy, and in Verocchio's studio in Florence he had some experience with fountains, but it was during his time in Lombardy that he discovered possible applications that were unknown in Tuscany. Advanced canal building had long existed in Lombardy, for hydrographical reasons, and as the Duke's engineer, Leonardo had to apply himself to the fundamental problem of water since not only agriculture but also the working of machines and mills depended on its being properly regulated.
By carefully observing the flowing of rivers, he drew a number of conclusions about movement, erosion, and currents on the surface and below, often with the assistance of little wooden or glass models through which he made water flow. The results of these experiments were then applied to the practical problems of canal building, and many drawings of bulkheads, portholes and locks with movable gates are preserved in his notebooks. Leonardo's mind then turned to the much greater project of intervening in nature by deviating the River Arno and building a large, navigable canal which would connect Florence to the sea, through the area of Prato, Pistoia and Serravalle, and provide the longed-for access to the Tyrrhenian Sea. He devised further ambitious hydraulic schemes for the Veneto, where he saw the possibility of flooding the Isonzo valley in the event of a Turkish invasion, and for Lazio, where Pope Leo X consulted him about draining the Pontine Marshes.
Fascinated by the idea of enabling men to move on and under the water, he designed buoys and breathing equipment, as well as considering the possibility of making boats move faster, with an improved hull in the shape of a fish or by machines operating paddles from within the vessel. The requirements of war persuaded him of the usefulness of a vessel with a double hull to protect it from the kind of damage he proposed to inflict on enemy ships, with men in diving suits equipped with screw devices breaking the keel and stealthily anchoring the ship to the bottom of the sea. Supplying city centres as well as the need to drain basins and marshland involved Leonardo in improving machines and pumps which had been known since ancient times. The Archimedean screw and water wheels fill the notebooks of Francesco di Giorgio Martini and other engineers of the time but, with Leonardo, they attained a graphic and functional perfection unknown elsewhere.