Final Answer?
In 1772, James Cook set out on his second voyage looking for the mythical Terra Autralis, the great southern continent which had been described originally by Ptolemy. Before the Portugeuse successfully rounded the tip of Africa, it was believed that this enormous continent wrapped around the end of the world, connecting Africa with Asia and making the Indian Ocean an inland sea. Throughout the period of discovery that followed the opening up the eastern passage to Asia, explorers such as Balboa, Magellan, Drake, and others contributed to the overall scaling down of hypothetical continent. Cook's voyage put to rest once and for all the idea that there was a vast habitable continent to the south. He came within 75 miles of the coast of Antarctica -- the much smaller, ice-covered continent that really was there -- but left discovery of it to a pair of Russians, Mikhail Lazarev and Fabian Gottlieb von Bellingshausen, in the 1820's.
I wonder whether the mission to Pluto just launched might serve to provide a final answer to the question of whether Pluto is, indeed, the ninth planet? In a decade or so, thanks to what this spacecraft shows us, Pluto may be vindicated as a planet or reclassified an asteroid or trans-Neptunian object. In the latter case, the Ninth Planet will join the great southern continent, the realm of Prester John, and myriad other fanciful notions of what lies beyond the horizon that have had to be abandoned in the harsh light of discovery.
Personally, I'm rooting for Pluto to keep it's status. But if it is reclassified, it had a pretty good run as a planet. Not as long a run as the southern continent, perhaps, but then things happen faster nowadays.