There were many fantastic engineering achievements by the Inca people. The Great Inca Road can be considered their greatest engineering achievement.
These networks of roads made the four regions connected, spanning over 25,000 miles (6 South American countries in today's geographical landscape). To give a mental image of how long these roads spanned, one could circumnavigate the world in that many miles!
Not only was this an engineering achievement in relation to the sheer span of road laid down, it was also impressive because the landscape was so different along these 25,000 miles. The engineers were innovative in their design of the roads. They used containment walls, dirt support, paved stones, and stairways to ensure that the roads were suitable to the terrain and sustainable to weather and time.