The tunnels were used as stone quarries for the buildings in Paris (limestone and also gypsum to make plaster in the North East of Paris and moder Seine Saint Denis): above the quarries there used to be fields and pastures (at the end of the XVIIIIth century, Paris was generally within the perimeter of lines 2 and 6 of the modern metro network) but with urban expansion in the XIXth century quarries crumbling down causing structural failures in buildings above them (and occasionally many deaths). This lead to the creation of the General Inspectorate of Quarries (IGC) at the end of XVIIIth century, which still exists to this day.