The Mine Passage is accessed from Capybara Plantation by using 4 Keys of Capybara on the marker stone. In the same way, 4 more keys are needed to open the marker stone in this map to go to 3F where Hell Breaker resides.
Here is an annotated map of the Mine Passage...
A pioneering memorial (v2.6) has been added at B7. The Treasure Chests are the same as those in Capybara Plantation.
Underground Lockers contain the Mine Diaries, quest items needed for one of the Toubkal Quests.
Lockers can be opened if you have Key Chain of Mine (see Toubkal Quests). They give a random item, such as Lv.84 Enchantment Chip, Lv.90 EXP Card x3, Diamond Coupon, Mystic Amber/Spinelle x5, or Resurrection Potion x20.
Each Electric Tin Mine drop 1-5 Pebbles (Pieces of Turmaline) on the ground for anyone to pick. Once emptied, each tin mine refills itself with pebbles after 3 hours.
The Golden Bats (Lv.108) are the same as those in Capybara Plantation, except they are tougher.
The mini-boss Gullfaxi (Lv.103) looks like the Golden Bat, but its attacks are different. It has ranged attacks, so the trick to fight the Golden Bat (see Capybara Plantation) does not work on it. It can drop Pure Gold Bars and Lv.92-96 Enchantment Chips.
The normal monsters in Capybara Mine Passage drop Mysterious Steel Piece, Key of Capybara, and HQ Emerald. It is also said that Gigantic Celegrimworm (Lv.108) drop Old Journal #10.
Related IAHGames Forum Links - Capybara Guide for the Lost ☼ Into The Realm Of The Golden Bat ☼ Key Chain of Mine ☼ Old Journal #10 Secret