20,000 Leagues Under The Sea.
Fallout 4: Far Harbor is supposed to be the biggest bit of post launch content that Bethesda have ever worked on. yes, bigger than Dragonborn. Yes, bigger than Mothership Zeta. Yes, bigger than The Shivering Isle. It’s a major expansion, and players are doubtless excited for what all it may add to the base game.
‘What it may add to the game game’ seems to be a lot- on Reddit, some people data mined Fallout 4 files, and found indication of a lot of new kinds of enemies that may be added to the game- these include Ghoul Yao Guai and Giant Squids. The data mining also found references to the harpoon gun that was leaked for the game a while back, indicating that water combat may be making an appearance in Far Harbor. it is speculated that Far Harbor may in fact have a full Rapture style underwater city, too.
Far Harbor is due out on PlayStation 4, Xbox One, and PC in May, so we should we able to lay all speculation to rest soon.