For Inca Road Builders, Extreme Terrain Was No Obstacle : Parallels : NPR

A new exhibit at the National Museum of the American Indian highlights the engineering prowess of the Inca, whose great road once spanned mountains, deserts and forests in 6 South American countries.

http://www.npr.org/sections/parallels/2015/08/29/435480149/for-inca-road-builders-extreme-terrain-was-no-obstacle