First of all, I'll start off with simply saying that MMO-Champion has suggested that 'The Dark Below' is a hoax after spending 24 hours researching and trying to find more proof beyond the registering of the name. They can't find any.
And now onto the more interesting part: Blizzard has given us a preview of the Timeless Isle, released in 5.4. Now, I've already gathered a lot of info from Cymre's blog Bubbles of Mischief in her time on the PTR, so a lot of what Blizzard didn't want to spoil has already been spoilt (entirely my own fault for reading the posts), but they've gone on to explain a few things I didn't know, like events and such, and the fact that, if you are within range of rares or rare chests, they show up on your mini map - though, just because you know where they are, or can even see them, doesn't mean they'll be easy to get to. For full details check out the blue post.
Also, if you haven't already, get your Gahz'rooki's Summoning Stone from the Battlefield Barrens weekly. Battlefield Barrens will be leaving soon, probably around the time the next patch comes out, which is rumoured to the the 27th of August.