After you beat lord Seymour again in the Ronso's mountains, what do you do in that cave? I hit the circle with Wakka's ball and it lit up and gave me a treasure chest. So thats where I'm stuck at if you know what to do after that part plz tell me, I'm stuck.

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After beating seymour again, you have to go swimming. There will be another trial. I'm not sure but I think stairs open to the north. This time you have to press three buttons. Tidus presses the blue betton. Riku presses the green, and Wakka presses the Orange. After that a new path opens to the east. Just proceed and you're out. Be prepared though a status attack boss is up ahead.