To explore the underwater ruins in Language of the Ancients in Boetia in Assassin’s Creed Odyssey swim over the the partially standing temple and pick up the key from the skeleton.

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Pick up the key from the skeleton to explore the underwater ruins in Language of the Ancients in Boetia in Assassin’s Creed Odyssey

Swim over the broken wall and pick up the stele piece from the chest.

Next: Explore the Coastline to Discover the Underwater Temple