Machu Picchu is situated above a loop of the Urubamba River,
which surrounds the site on three sides, with cliffs dropping vertically for
450 metres (1,480 ft) to the river at their base. The area is subject to
morning mists rising from the river. The location of the city was a military
secret, and its deep precipices and steep mountains provided excellent natural
defenses. The Inca Bridge, an Inca rope bridge, across the Urubamba River in
the Pongo de Mainique, provided a secret entrance for the Inca army. Another
Inca bridge was built to the west of Machu Picchu, the tree-trunk bridge, at a
