It can be unlocked after completing Chapter 10. In Explorers of Time and Darkness, it is unlocked by accepting an escort mission available after reaching Bronze Rank. In Explorers of Sky, it is instead unlocked by finding its location at the bottom of a drink in Spinda's Café.
Gameplay
The jobs here are ranked E-C by default, depending on the floor.
The floors are large sized, with 3-7 rooms on floors 2, 4 and 7, 3-10 rooms on floor 3, 3-12 rooms on floor 1 and 5, and 3-14 rooms on floor 6. There's 1-4 items on each floor, and 2-3 starting enemies. Monster Houses are a frequent occurrence.
There is rain on floor 3, cloudy weather on floors 4 and 5, and fog on floors 6 and 7. The turn limit is 1000. As this is a water dungeon, Drought Orb will fail if used here.