In other words, they want to build the fabled city of Atlantis.
Except without
- the ability to safely sink below the ocean and rise up again at will. For example, to submerge beneath passing tropical storms.
- the ability to travel the seven seas to travel and explore [ok, my idea, not part of the fable, unless you consider below]
- the ability to take off and fly between star systems and/or galaxies [the Stargate variety]
Of course, should tragedy strike and it really does sink (assuming its not designed to do so), then it really would fit the story of the real ancient city of Atlantis (presumably an island that was hit by an earthquake and sank below the ocean) that inspired said fables.
Joking aside, this is a good idea for the future. Particularly for Japan, which suffers from a shortage of stable land, and whose people are already used to commuting between islands. Furthermore, if they can truly make it self-sufficient, then it would serve as a great model to learn how to build future Lunar,Martian,etc. colonies.