"Alpha". 200x200m Sea-Dome and Underwater complex adjacent to DeathSaucer 'Omega'. Build currently in progress. Dome is now sealed. Entrance is proximity based west and east faces. Approach the glass along the center-most point. North and South faces will take you to the saucer and the top of the dome respectively. Command blocks are still using 'testfor' so this will not work in 1.13 (I'll adapt the code once it's available - and if they resolve some issues with underwater visibility based on the snapshots). Central tower takes you to the underwater build which is just getting started. It's excavated down to bedrock from just below the ocean floor. Once the city is built - the ocean floor will be removed to allow the ocean to enter.