Leonardo da Vinci kept plans to get rich on sea ransoms a closely guarded secret for much of his life - but eventually scrapped the idea when his conscience got the better of him.
The new window on Leonardo's thinking - and his previously unknown mercenary urges - has been opened by fresh readings of two crucial Leonardo papers, the Codex Atlanticus and the Codex Hammer.
In the first, explains the curator of a new show here, Leonardo expert Alessandro Vezzosi, "we see a diving bell which would have revolutionised sea warfare and allowed him to enrich himself on ransom money".
Throughout the history of sea warfare until the late 19th century, prisoners of a certain rank or status were allowed to buy back their freedom.
In the unrealised diving-bell scheme, divers wearing glass goggles and thick waterproof trousers would have lowered themselves to the sea bed with sand bags and then floated their crafts up to sink ships by punching holes in their keels.
Leonardo would have been entitled to half the ransom money from nobles sunk by his invention, Vezzosi explained. Next to the design in the Atlanticus, Leonardo scribbled "do not teach this and you will be the only excellent one" - codewords, Vezzosi believes, "for keeping the intellectual rights so that he alone would benefit from the ransoms paid by wealthy prisoners".
But the Renaissance genius never acted on his plans and 20 years later, in the Codex Hammer, explained why he had second thoughts about his earlier scheme to rake in much-needed cash.
The artist and inventor reveals that he kept this and other submarine ideas to himself "to prevent blood flowing across the bottom of the seas".
"The story of Leonardo's little-known invention reveals how the artist was initially lured by the prospect of riches but in the end repented and rediscovered his profound ethical sensibility," said Vezzosi, who is director of a Leonardo museum at Vinci near Florence.
Because of his famously restless intellect, Leonardo left several of his grandest artistic projects unfinished and made no money at all from his scientific ideas. Unlike his near-contemporary Michelangelo, he was hopeless with money and forever being fobbed off by wealthy and powerful patrons.
He died a virtual pauper after living for years off the generosity of his last host, Francis I of France.
During its heyday in the 16th century, landlocked Florence fought several naval wars, mostly against its powerful neighbour Pisa. Leonardo, Myth and Truth runs at this Tuscan spa until October 10.
