Leonardo da Vinci and Niccolò Machiavelli once teamed up to steal the Arno river.
Leonardo da Vinci was fascinated by the Arno from a young age. He was born in the valley around the river, and his earliest dated drawing is a sketch of the Arno Valley. That drawing, by the way, is sometimes credited as the first genuine landscape in Western art. Previously natural terrain had shown up in the background of religious scenes, but this was supposedly the first time someone paid attention to the landscape itself, for itself. Da Vinci had reportedly floated the idea of making the Arno navigable so that ships could travel from inland Florence to Pisa on the coast, an idea that would become useful a few years later.
In the early 16th century Florence was at war with Pisa. To be fair, Florence and Pisa were at war quite a lot: Pisa had been under Florence’s control from 1406 until 1494, when Pisa broke free as the Second Pisan Republic. This time Pisa was in real trouble, because Florence had the huge advantage of two of the greatest minds of Renaissance Europe: Da Vinci and Niccolò Machiavelli. And these two cooked up an audacious plan to break the power of Pisa. They would steal its river.
Now, I should mention that we only have circumstantial evidence that these two plotted together. It sounds like pretty strong circumstantial evidence, though – they were both involved in the scheme so it would be incredible if they hadn’t colluded somewhere along the way.
I imagine Machiavelli as the ideas man in this heist. The Arno travels through Florence on its way to Pisa, so if the Arno could be redirected then Pisa’s supply of fresh water would be gone and Florence would have valuable irrigation and their own navigable waterway leading to the sea. It would be a bloodless geopolitical power grab of huge proportions. Da Vinci was the designer: he set out a detailed plan for how to achieve this involving canals, tunnels (under a mountain, no less), and a huge workforce to build them.
Alas, the heist was not successful. Apparently the engineer in charge of the project skimped on the designs and the channels never got large enough to divert the Arno. In any case Pisa was conquered by Florence in 1509 and the whole plan was abandoned.