How do you get into posiedens lair on Mythology Island?

To access Poseidon's Lair on Mythology Island in Poptropica, you first need to complete the challenges on the island, including obtaining the Golden Harp from the Muse. After collecting the harp, head to the underwater entrance located near the ocean. Use the harp to summon Poseidon, who will then grant you access to his lair. Be prepared to solve puzzles and interact with various characters inside the lair to progress in the quest.