A drawing of a map covering about 40 miles (65 km) of the coast south of Rome, with north to upper left, and depicting the plain of the Pontine marshes. Melzi's number 169.

In 1514 Pope Leo X charged his brother, Giuliano de’ Medici, with the task of draining the malarial Pontine marshes, to the south of Rome. Giuliano was Leonardo’s patron in Rome, and this map demonstrates that Leonardo – by then a respected engineer and mapmaker – was involved in the project in some capacity. The scheme was simple, to cut two channels through the marshes to the sea. Digging began in 1515, but the populace of the area resented what they saw as papal annexation of their lands, and work halted with the death of Giuliano in 1516. The marsh was not fully drained until the 1930s, and still requires constant pumping.

Text adapted from Leonardo da Vinci: A life in drawing, London, 2018