The mines predate the foundation of the pony kingdoms and their true origin is unknown by pony-kind.
While the upper levels are filled with Ghost Sulfur, used to mark areas of dangerous taboo in Blackmist March, the lower mine shafts are filled with materials and relics of immense value. However, the walls of the mines are littered with massive blocks of unbreakable ice which encase the perfectly preserved bodies of ponies from some unknown era.
The mine's mouth has been marked off by the same Ghost Sulfur torches which are extracted from its upper levels. The longer anypony remains in the mine, the more they claim to feel that they are being watched. Spending more than a couple hours in the upper shafts has caused ponies to begin hallucinating, seeing small figures darting between the shadows even when they are completely alone.
The lower levels of the mine are a death sentence. Nopony in Fansie history has been able to plum the depths of the lowest levels and returned with their minds in a state fit to ever tell what they saw, and many more have simply never returned from their voyages.
Because of these risks, Arcturus de Chevalier made one of his first acts as the newly enstated Marquis de Chevalier to close the mines from public access.
To this day, residents of Blackmist March are forbidden from entering the mines, except on business for Maison Chevalier or the Fansie Royal Family. This permits approved ponies to enter and collect Ghost Sulfur from the upper levels when there is need of it.