In world War II, an Italian Auxiliary ship off the coast of Gibraltar was said to be a staging area for Italian Frogmen acting as human torpedoes. The Olterra, to the plain eye, was just a civilian ship with civilian crew. Underneath, however, a trap door entering into the water would open and the Italians would swim and place limpet mines on Allied Ships undetected. The Olterra would also act as a observation post.
In Thunderball the novel, Ian Fleming needed Emilio Largo to have a way for the Disco Volante to be docked so close the the Nassau harbour, yet a way to get the bombs to the USA undetected. The Olterra was the perfect fit so like all good fiction writers he nicked the idea and wrote about what he knew.
13 / 'My Name Is Emilio Largo'
'Same as you. There's at least half of that ship we didn't see. But then again there's a perfectly good answer to that. He may have got a stack of secret treasure-hunting gear down there he doesn't want anyone to see. Remember that merchant ship off Gibraltar during the war? The Italian frogmen used it as a base. Big sort of trapdoor affair cut in the hull below the waterline. I suppose he hasn't got something life the?'
Bond looked sharply at Leiter. 'The Olterra. On the the blackest marks against Intelligence during the whole war.'
14 / 'Sour Martinis'
Now Bond's mind was made up. It was the hull of the ship he wanted to have a look at. Leiter's mention of the Olterra had been a long shot, but it just might pay off.