Dark Ore Mine
The Dark Ore Mine is located within the Fire Realm. It is an area from which Gorons excavate the valuable Dark Ore, which is susceptible to sunlight and melts quickly. Link can purchase Dark Ore from the Dark Ore Salesman and can transport the material as part of a sidequest.
The area is unlocked by transporting Ferrus to Aboda Village. After this is complete, Link will receive a letter from Ferrus asking for his service once more. Link must pick up Ferrus at the Fire Realm and drive him to the Ocean Temple in order to receive a Force Gem.
Linebeck III asks Link to transport five pieces of Dark Ore back to his station. Link must brave a tunnel containing a Rocktite to accomplish this, however. Upon successful completion of the task, Link is awarded with a Force Gem, which unlocks the Lost at Sea Station located within the Ocean Realm.