Machu Picchu is encircled by the Urubamba River (pictured), considered sacred by the Inca.

Nestled atop a mountain ridge in Peru, the 15th-century Inca city of Machu Picchu had sat largely forgotten for centuries—until archaeologist Hiram Bingham began excavations of the ruins a hundred years ago this week.

Now one of the most popular tourist destinations in the world, Machu Picchu’s original purpose is still unknown—though many archaeologists think they are closer to finding an answer.   (Take a Machu Picchu quiz.)

Here are some of the top theories about Machu Picchu proposed—and in some cases disproven—in the century since its “rediscovery.”

to read more, go to:    http://news.nationalgeographic.com/news/2011/07/110721-machu-picchu-100th-anniversary-archaeology-science/