The Giant Pearl is one of the sacred items that Zeus asked you to find to save Poptropica and to become immortal.
How to get the Giant Pearl
Dive down underwater you can use the bubbles in the water for additional air. The way you want to go is across the top, down the right, and then left on the bottom. Soon you’ll get to the entrance to the Hydra’s lair. Before you go into it, get the giant pearl inside the clam that’s above and to the right of the cave entrance.