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Special Charter Shaft is one of the deepest areas of the Henne Mines. To unlock the path to this zone, first you must defeat at least 10 Espers. Then, go to Jahara and talk to the Geomancer sitting on the little hill in Lull of the Land. He will unlock the gate in the cavern just north of the orange save crystal in the mines. Equip Ribbons if you got 'em, and be careful down there! Excalibur is your best friend in this area.