In the realm of Poseidon, there is a shallow sea with a beach. An area of darker water marks the location of the underwater maze that leads to the Hydra cave. Just outside is the Giant Clam that has the Giant Pearl, another of the 5 sacred items.
you find it in the giant clam in the water where it says dive. :)
You have to go underwater which is left to Aphrodite and you have to find a clam. It opens and closes. Try to get the pearl (which is inside the clam) when the clam is open.
go to posidens lair and go till u find the clam. be sure to collect air bubbles
On the island
You can find them near water or in water.
Mythology island, on a statue in the museum
Find him and click on him.
Just stay along the top, along the right side, then along the bottom until you reach the Hydra cave. The Giant Pearl clam is just above the entrance.
You have to get it from a "many headed" dragon
You can find her when you first enter Mount Olympus with Hercules.
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under they sea at Triton's palace