Crossbone Isle

From Golden Sun - Wiki on Neoseeker

Crossbone Isle is a hidden island that does not appear on the map in the overworld. You can only get access to it by being sucked up my the giant lizard in the Suhalla Desert. This dungeon is filled to the brim with different trinkets and treasures, and even holds place to the (presumed) hardest boss in the game; Deadbeard. However, you will not fight him till you reach the end.

This place is filled with different puzzles and traps, it's almost a pain to get through. You also have to deal with moderately strong monsters randomly attack you from time to time.