WASHINGTON (Reuters) - The only surviving copy of the 500-year-old map that first used the name America goes on permanent display this month at the Library of Congress, but even as it prepares for its debut, the 1507 Waldseemuller map remains a puzzle for researchers.

A detail of the map that first used the name "America" is seen at the Library of Congress in Washington, December 3, 2007. The 500-year-old map, created by German monk Martin Waldseemuller, is the only known surviving copy and was purchased for $10 million in 2003. REUTERS/Jim Young

Why did the mapmaker name the territory America and then change his mind later? How was he able to draw South America so accurately? Why did he put a huge ocean west of America years before European explorers discovered the Pacific?

“That’s the kind of conundrum, the question, that is still out there,” said John Hebert, chief of the geography and map division of the Library of Congress.

The 12 sheets that make up the map, purchased from German Prince Johannes Waldburg-Wolfegg for $10 million in 2003, were mounted on Monday in a huge 6-foot by 9.5-foot (1.85 meter by 2.95 meter) display case machined from a single block of aluminum.

The case will be flooded with inert argon gas to prevent deterioration when it goes on public display December 13.

Researchers are hopeful that putting the rarely shown map on permanent display for the first time since it was discovered in the Waldburg-Wolfegg castle archives in 1901 may stimulate interest in finding out more about the documents used to produce it.

The map was created by the German monk Martin Waldseemuller. Thirteen years after Christopher Columbus first landed in the Western Hemisphere, the Duke of Lorraine brought Waldseemuller and a group of scholars together at a monastery in Saint-Die in France to create a new map of the world.

The result, published two years later, is stunningly accurate and surprisingly modern.

“The actual shape of South America is correct,” said Hebert. “The width of South America at certain key points is correct within 70 miles of accuracy.”

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Given what Europeans are believed to have known about the world at the time, it should not have been possible for the mapmakers to produce it, he said.

The map gives a reasonably correct depiction of the west coast of South America. But according to history, Vasco Nunez de Balboa did not reach the Pacific by land until 1513, and Ferdinand Magellan did not round the southern tip of the continent until 1520.

“So this is a rather compelling map to say, ‘How did they come to that conclusion,’” Hebert said.

The mapmakers say they based it on the 1,300-year-old works of the Egyptian geographer Ptolemy as well as letters Florentine navigator Amerigo Vespucci wrote describing his voyages to the new world. But Hebert said there must have been something more.

“From the writings of Vespucci you couldn’t have prepared the map,” Hebert said. “There had to be something cartographic with it.”

MISGIVINGS ABOUT AMERICA

Waldseemuller made it clear he was naming the new land after Vespucci, describing how he came up with the name America based on the navigator’s first name.

But he soon had misgivings about what he had done. An atlas Waldseemuller produced six years later shows only part of the east coast of the Americas, and refers to it as Terra Incognita -- unknown land.

“America has gone out of his lexicon,” Hebert said. “(No) place in the atlas -- in the text or in the maps -- does the name America appear.”

His 1516 mariner’s map, on the same scale as the 1507 map, steps back even further, showing only parts of the new continents and reconnecting the north to Asia. South America is labeled Terra Nova -- New World -- and North America is labeled Terra de Cuba -- Land of Cuba.

“Essentially he’s reconnecting North America to the Asian mainland, suggesting a continual world of land mass rather than separated by those bodies of water that separate us from Europe and Asia,” Hebert said.

Why the rollback? No one knows.

In writings accompanying the 1516 map, Waldseemuller comes across as if he “has seen the better of his error and is now correcting it,” Hebert said.

He speculated that power politics played a role. Spain and Portugal divided the globe between them in 1494, two years after Columbus, with territory to the east going to Portugal and land to the west to Spain.

That demarcation line is oddly absent from the 1507 Waldseemuller map, and flags marking territorial claims in South America suggest Portugal controls the region’s southernmost land, even though it is in Spain’s area of influence. On the later map, the southernmost flag is Spanish, Hebert said.

“It is possible one could say the 1507 map is influenced strongly by Portuguese sources and conceivably the 1516 map may be influenced more by Spanish sources,” he said.

Although the map conceals many mysteries, one thing is clear: it represents a revolutionary shift in the way Europe viewed the world.

“This is ... essentially the beginning or first map of the modern age, and it’s one that everything builds on from that point forward,” Hebert said. “It becomes a keystone map.”