First mentioned in Day 189.
It's placed in the Labyrinth City Aquarium.
One of the dungeon's basic specifications is that it's made up of underground levels. In addition, many of its traps are water-based, and the monsters that appear there are primarily those of the elemental, fish, and shellfish systems.
The final layer is 50 floors down, and in the dungeon's long history, there's been very few stories of adventurers making it to that floor and successfully killing the boss.
Most of them aren't capable of defeating the boss enshrined on a specific level. Since this prevents them from delving deeper, it looks like these people enter the upper levels of the dungeon every day in order to collect items and earn daily cash income.