A sanctuary under the waters of the Dead Lake, protecting a tremendous relic: the third and final artifact in the Moon Manipulator / Book of Doom combo! Can only be reached by using the Crystal Key to unlock a door in the Crystal Palace, which leads to the Sapphire Door - which, when opened by the Sapphire Key, is the only door into the Sunken Temple.

If you went under the waters of the Dead Lake, you would not find this temple - it can ONLY be reached via the Sapphire Door.