Complex sentence: A citizens'association argued forcefully against the planned demolition of the surface installations and filling in of the historic mining areas, so that the disused mine became a unique museum, which became a UNESCO 1992 World heritage project with Goslar's Old Town.
Split into simpler sentences: A citizens'association argued forcefully against plans to demolish the surface installations and fill in the historic mining areas. Consequently the disused mine was developed into a unique museum, which became a UNESCO 1992 World heritage project together with Goslar's Old Town.