On September 25, 1513, Vasco Nunez de Balboa, from a bare peak in the Serrania del Darien in what is now Panama, saw water to the south. He had been told about it by local people, and he had climbed that morning so that he might be the one to see it first. He called it the South Sea and claimed it, and the lands draining into it, for the Crown of Castile.
He had no business there. Balboa had stowed away on an expedition in 1500, had ended up in Hispaniola, had failed at farming, and had hidden in a barrel of salt pork on a ship sailing to the mainland to escape his creditors. The settlement he ended up in, Santa Maria la Antigua del Darien, was failing when he took it over in 1510 and sent to Spain for help. He was also, technically, a rebel, because the man who had appointed himself governor was under arrest at the time.
The expedition was small. It had about 190 Spaniards, some of them the survivors of a colony that had already failed, and about 1,000 Indian porters and fighters, some allied and some not. They crossed the isthmus in about 24 days, through swamp and forest and over the mountains, and the distance was only about 60 miles as the crow flies.
What made it possible was Indian help and Indian knowledge, and what made it possible to get back was the same. The gold the Spanish had been given and taken on the way is what drew them, and the reports of a rich country to the south, which is what Peru turned out to be. Balboa also had a reputation for a particular kind of cruelty: he was said to have fed caciques who resisted to his dogs. He did it in public.
The discovery did what Columbus never managed to do, which was to show that there was another ocean between the Americas and Asia. It also made Balboa's position worse rather than better, because it drew the attention of the crown to a man who had been governing without authority. Ferdinand sent Pedro Arias Davila, Pedrarias, as governor in 1514 with a fleet of some 1,500 people, and Pedrarias spent the next four years obstructing him.
In January 1519 Pedrarias had Balboa arrested on a charge of treason, and he was beheaded at Acla in Panama, aged about 43. Magellan, who had been in the expedition's company and who sailed in 1519 to find the passage to Balboa's sea, named it the Pacific on the far side of the world.