Machu Picchu is situated above a bow 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 location where a gap occurs in the cliff that measures 6 metres (20 ft). It could be bridged by two tree trunks, but with the trees removed, there was a 570 metres (1,870 ft) fall to the base of the cliffs.