So i noticed that Shell Island secret area introduce in BOTH main story and sidequest
-The main quest one is where u meditate inside the cave thing (goodluck leaving the cave)
-The sidequest one is where u touch the box inside the cave (still goodluck leaving the cave)
So this could imply that the cave probably have something veri important to lore cuz the introduce TWICE in game unlike others secret area in Bronze Sea.
Conclusion: Shell Island Cave is abandoned laboratory that create giant bananas
Would be cool to see sidequest’s reward replaced from powerful scroll to spirit weapon later (Someone had this idea in the topic about spirit weapons).