A crystal-mine key (steel key ring with the key added works) is required. Climb down the stairs found on the surface at the north-east part of the Abandoned Mine, open the large doors to the west. Mine the mithril ore vein on the wall. If this doesn't work, start in Burgh de Rott. Go west and hop the fence (quest required). Go down the stairs into the mine. There will be doors to the west and doors to the east. Go through the doors to the west (quest may be required). The mithril ore is on the north wall just past the doors which were just passed through.