It was precisely for this reason that the Merfolk guarding the area would occasionally come over to open a breach in the bubble, using water to submerge the sandy ground within the bubble.
The bubble was specifically designed by the Merfolk to imprison those humans who had trespassed into the Great Whirlpool.
The reason they captured and detained humans was actually for their own reproduction. Although the Merfolk and Goblins had strikingly different appearances, their methods of reproduction were quite similar.
The difference was that there were no females among the Goblins, so they had to capture other female humanoid creatures and use their bodies to reproduce.
Although the Merfolk had both males and females, the strappingly handsome Merfolk that appeared on the outside were actually all female; there were no male Merfolk in their race.
