Fallout 4's $25 Far Harbor add-on is the most substantial DLC the game has had so far. In fact, Bethesda affirms that Far Harbor is the biggest DLC the team has ever made. It takes place in the foggy, storm-ridden irradiated coasts of Bar Harbor, Maine, and the landmass is actually bigger than Oblivion's Shivering Isles. As far as specifics, Far Harbor features a ton of content to chew on, including new main, side, and faction quests, horrendous monstrosities, armor and weapons, settlements, and more.
In the trailer we also get a glimpse at what appears to be an old-fashioned diving suit, possibly confirming underwater exploration. Far Harbor will also have a considerable Children of Atom sect that worships in a working nuclear power plant. The region features a number of environmental factors, including heavy volumetric fog (possibly think enough to get lost in) as well as steady rainstorms and tumultuous waves. Post-apocalyptic Maine is also riddled with extreme radiation, and in some areas the rads are so bad you could probably die within 10 seconds or so. The radstorms are much more dangerous, too. The overall atmosphere is one of dark, stormy dread with overtones of eerie sentiments from John Carpenter's The Fog.