The Deepsea

Or The Sea That Should Not Be.  Created in Phase 3 by The Phoenix Queen.   A clay-sealed underground sea 1 mile under the surface of the planet in a very difficult to spot pocket dimension (blends in perfectly with everything else, but basically doesn't delete or displace any existing mineral deposits or caverns). Due to being clay-sealed, it is entirely self-sustaining as an ecosystem and based around chemo-synthetic life. I create super-effective chemosynethetic algae for the sea and river surfaces, to serve as the basis for this new ecosystem. Any chemo-synthetic life that can survive here, I import copies of to grow and flourish.   This does not undermine/collapse any mountains or foundations. Pre-discovery, I set the seas to hyper-abundance and hyper-evolution (to generate some unique cave fauna). I also create chemo-synthetic non-light-dependent bio-luminescent corals for one particular corner of the big sea. These blend naturally into the cavern walls and can also grow like barnacles in the air above the sea To access this space, you just need to dig into the layer where it exists I would also like to put some Chitin Trees there: basically, floating trees made of multi-colored chitin-like bark (and with soft leaves with sticky sap used for catching bugs) that drift along on root-rafts that are made unusually sealed and bouyant by tar-like sap. Also any pirate ships that ever make it down here evolve at comically fast rates to adapt to their environments. Not that I think they will