<P> Heavy rainfall required terraces and stone chips to drain rain water and prevent mud slides, landslides, erosion and flooding . Terraces were layered with stone chips, sand, dirt and top soil, to absorb water and prevent it from running down the mountain . Similar layering protected the large city center from flooding . Multiple canals and reserves provide water throughout the city that could be supplied to the terraces for irrigation and to prevent erosion and flooding . </P> <P> The Incas never used wheels in a practical way, although their use in toys shows that they knew the principle . The use of wheels in engineering may have been limited due to the lack of strong draft animals, steep terrain and dense vegetation . The approach to moving and placing the enormous stones remains uncertain, probably involving hundreds of men to push the stones up inclines . A few stones have knobs that could have been used to lever them into position; the knobs were generally sanded away, with a few overlooked . </P> <P> The Inca road system included a route to the Machu Picchu region . The people of Machu Picchu were connected to long - distance trade, as shown by non-local artifacts found at the site . For example, Bingham found unmodified obsidian nodules at the entrance gateway . In the 1970s, Burger and Asaro determined that these obsidian samples were from the Titicaca or Chivay obsidian source, and that the samples from Machu Picchu showed long - distance transport of this obsidian type in pre-Hispanic Peru . </P> <P> Thousands of tourists walk the Inca Trail to visit Machu Picchu each year . They congregate at Cusco before starting on the one -, two -, four - or five - day journey on foot from kilometer 82 (or 77 or 85, four / five - day trip) or kilometer 104 (one / two - day trip) near the town of Ollantaytambo in the Urubamba valley, walking up through the Andes to the isolated city . </P>

What is macchu pichu when was it discovered and why did it take so long to find it