What is a specially perpared battle line running behind the German trenches in WWI?

The Hindenburg Line was a vast system of defenses in northeastern France during World War I. It was constructed by the Germans (using Russian POWs as labor) during the winter of 1916–17. The line stretched from Lens to beyond Verdun. A portion of the line was known as the Siegfried Line, not to be confused with the better known Siegfried Line of the Second World War