It's been discovered by multiple sources that Blizzard recently trademarked the term "The Dark Below" to indicate, amongst other things, a computer game.
Is this the new Warcraft expansion? Or the Diablo 3 expansion? Or something else entirely. We have zero idea of what this actually is, but hey -- that doesn't mean it's not something to look at and speculate on. It is interesting that "The Dark Below" exists in WoW via Blizzard's non-canonical RPG manuals and via a brief mention with Ozumat.
So start your engines ... something is happening.