The Dark Depths is just beyond The Great Maw in Hollow Bastion. Several cutscenes are shown at the Dark Depths such as Axel confronting Sora and tells him of the Organization XIII's plans with Kingdom Hearts when Saix warps in to try and capture Axel. Once the MCP has been defeated and the place has been named Radiant Garden, Sora can find and battle Sephiroth here. You can see the Villain's Vale where Maleficent used to reside from the Dark Depths. In Kingdom Hearts, the area where you fight your first Behemoth and seal Hollow Bastion's Keyhole is also called Dark Depths.