I think I found some sort of secret portal inside Zora's Domain:
When you enter Zora's Domain as a kid,climb down the latter to Link's left. Get into the water and swim to the back of the island that the Zora is standing on.
While your in the water, face the back of the Zora standing on the island and dive down. You'll notice to your left down deep there is a entry-way at the bottom of the slope.
I don't know what's inside but I have almost made it in. (NOTE:must have golden scale)
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