The Idrija mercury ore deposit originated 238 million years ago when hot solutions (160–210 ˚C) bubbled from the depths to the surface and brought mercury along with them. On their way to the surface, they converted older rocks into ore. Cinnabar ores originated at the same time as the rocks in the ore generation period. The cinnabar mineral is one of the forms of mercury, the second being elementary mercury. Both forms can be found in the Idrija mercury ore deposit – in terms of the quantity of mercury, it is the second largest in the world. Mercury is extremely pure and frequently occurs as drops of elementary Hg. In terms of its origin and geological structure, the deposit is extremely complex, and as such, it represents a globally relevant natural monument. Its special status is warranted by the diversity of cinnabar ores that can be found in the area.
Anthony’s Main Road is the oldest preserved part of the deposit still accessible after the closure of the mine. Today, it is a tourist attraction that features an impressive range of geological phenomena with elementary mercury and presents the methods of ore extraction used throughout the history of the mine.