Important items are one of the main stapes of The Legend of Zelda franchise. With every dungeon you complete, you will earn a trusty bit of equipment that will help you on your travel. One such item is the Bomb Bag.
To get the Bomb Bag, you must have reached the volcano region of Eldin. If you have not reached this area yet, you must progress into the story, and complete the first temple, to reach this fiery new area.
If you have reached Eldin, you need to navigate the area by solving the puzzles and then finding all five pieces of the temple key to enter the Earth Temple. Upon entering the Earth Temple, you must jump onto the rock platforms until you come to a drawbridge. On either side of the platform you are on, there is a cog on a gold platform you have to cut the rope. Then look toward to the right of the drawbridge, and you'll see the same cog. Use the Beetle gadget, and use it to cut the rope.
Once in the next room, kill the Lizardfos enemy just inside there, and hop onto the giant floating ball. Then move to the right, and you'll come to a platform with a cage on it. Crawl through the opening in the cage, kill the Bobokin inside, then roll a bomb through the opening in the cage you came from. This will destroy the rocks blocking the door. Now enter that door.
Inside this room, you have to kill two of Lizardfos creatures. Upon defeating them, a chest will appear, and inside is the Bomb Bag.
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