This area is unlocked after completing the quest: Deep in the Dark, available at level 19 or above, offers items available for crafting at Level 20. Items for collecting include glowing mushrooms and iron. You will need these items for future quests, crafts, and buildings. The cave area is accessible ONLY through the menu and map, there is not a gate entrance included with the cave. Darthy is the mole creature that inhabits the cave, and will offer quests along with the other main characters.