Viaducts (iron bridges) were constructed over the rail lines so people could get across easier. This gave rise to the a 'second level' which is where the city exists today. The original level (shown in this photo) would later become known as "Underground Atlanta." This photo was taken on Wall Street looking south down Central Ave. The Zero Mile Post can be seen just left of the wall in the right side of the photo, just left of what looks like a barrel.