You will need the Sol Blade to access the maze in the basement first floor. This is a link to the map:
A Jupiter Djinni, Lull, and Quality Zol can be found in the first room after using search to cause stone blocks to appear. The rest of the island is a maze and usually requires a map to cheat and find your way around. Alternatively you can use the 'right hand-rule' ('Pledge algorithm') , which means that you follow the walls on one side. This will lead you through the whole dungeons without problems. Hidden within is a Triton's Ward, Psy Crystal, Power Bread, and Mythril Silver.
This island is also the home of the Catastrophe Summon Tablet. It is guarded by the superboss Ancient Devil.