Colca Canyon
For a long time, it was believed that the Colca Canyon was the deepest canyon on earth, but it turned out that the nearby Cotahuasi Canyon is even deeper.
In any case, the Colca Canyon is an attraction and walking there is a unique experience.
It is quite a strenuous effort, however, - after all it is situated at a height of approximately 3500 meters and acclimatisation is essential.
More on the Colca Canyon at Wikipedia.
More pictures from Peru on my main site and in this blog:
http://peru.travelphoto.net/
http://www.travelphoto.net/a-photo-a-day/wordpress/category/photos/peru/