The Hidden Cavern is a small chamber in Deep Dome at the bottom of Deep Hole. It is blocked off by a boulder, and there is a lone Pygmy Sperm Whale trapped inside (the only one in Endless Ocean 2; it does not appear in Endless Ocean). This rock can be moved with the help of a saltwater dolphin partner, and the whale can be seen swimming around Deep Dome from then on.
The quest "Sealed Away" in the player's notebook says as follows:
"A chamber in Deep Dome in Deep Hole is sealed by a huge rock. If only you could move it aside..."
Once the player tries to move the rock alone, the notebook will update.
"It looks like you can't move the rock with human strength alone. If you team up with a sea creature might that be enough?"
As soon as the player gets help from a salt-water dolphin partner to move the rock aside, your notebook will wrap up the notebook entry.
"You moved the rock aside and released a pygmy sperm whale that was trapped inside!"
Completing this quest is the only way to unlock the Pygmy Sperm Whale that was trapped inside of the Cavern before the player moved the rubble blocking the entrance aside.