Machu Picchu was known to only a few Peruvian farmers until 1911, when an American historian named Hiram Bingham almost stumbled across it while searching for the lost city of Vilcabamba. Bingham found buildings thickly overgrown with vegetation. He thought at first he had found Vilcabamba, and he returned several times to dig at the site and try and solve its mysteries. Vilcabamba was later found to be much further into the jungle. Throughout the 1930s and 1940s, archaeologists from Peru and the United States continued to clear away the forest from the ruins, and later expeditions also attempted to solve the Machu Picchu mystery. Over 100 years later we still don't know much about the city. Current speculation is that the Incas had already deserted Machu Picchu before the Spanish arrived in Peru. This would explain why the Spanish chronicles do not mention it. One thing is certain. Machu Picchu has so many ornamental sites with exceptionally high quality stone works that it must have been an important ceremonial center at some point in Incan history. Interestingly, in 1986 archaeologists found a city larger than Machu Picchu just five kilometers north of the city. They have named this "new" city Maranpampa (or Mandorpampa). Maybe Maranpampa will help solve the mystery of Machu Picchu. For now, visitors have to come to their own conclusions as to its purpose.
Getting to Machu Picchu can be half the "fun". Most people go to Machu Picchu via the most popular route--fly to Cuzco, train to Aguas Calientes, and bus the last five miles to the ruins. The train leaves the Estación San Pedro in Cuzco several times daily (depending on the season and demand) for the three hour ride to Aguas Calientes. Some of the trains are express, others stop several times along the route. The local train can take up to five hours to make the trek. Hearty souls with more time can hike the Inca Trail, which is the most popular trail in South America. Backpackers should plan three or four days to trek the 33 km (>20 miles) route because of the high elevation and steep trails.
One added note for those traveling to Machu Picchu. The city has become an extremely popular tourist destination in the last few years, but its popularity is now endangering the environment surrounding Machu Picchu. Unplanned development is the culprit, and UNESCO placed Machu Picchu on its list of endangered World Heritage sites in 1998. Hopefully government officials can find a way to preserve this important cultural/archaeological site. For now, those who visit should respect the importance of the site and try and make sure they do nothing to further disturb the area.