Unlike sea, ocean, lake, or cave, I believe that there's no depth within the forest. Then, what does "deepest part of the forest" mean?
Native speaker’s answer
Rebecca
It means the same as it does with a sea, ocean, lake, or cave. It's the deepest and farthest away from the entrance to the forest as you can be. The depth of bodies of water is vertical. The depth of a forest is horizontal, which is why we say "deepest part of the forest." The depth of a cave can be vertical, but you can also walk straight through (horizontal) a cave.