This has be baffled like you wouldn't believe! Not because I don't have a clue... but because I do... I must be so close!! I don't think that English is your first language, hopefully you can decipher my rambling words.
As soon as I saw your shot I recognized it...almost. I've been coincidentally hanging out under Zul'Farrak lately exploring... and I was at a location just two nights ago that looked JUST like your screenshot... only without the water!! It was under the old Shimmering Flats part of Zul'Farrak off to the side, I was exploring on the top and the sides of the cave down there using Tigers and Blink.
So I see your shot with the water and figure, "oh, he must be in the real world Azeroth version of what I've been exploring"... so I head over to what used to be the Shimmering Flats, and of course it is underwater now, so I think that this is good and I am on the right track. Only I go under the world and go where I think you are... and I cannot find that particular location where you're standing on one of the angled walls and looking at the two straight walls. On the angled wall I am standing, there is only one straight wall in front of me. I was at least able to figure out that you are standing on an angled wall.
So I've determined that I am extremely close... I'm in pursuit and on the trail... I'll figure this out! NO HINTS, PLEASE! Just wanted to let you know my progress and that I am having fun. My guess is that it is either in the ZF instance and I missed the water, or it is very close by to the old Shimmering Flats where I already am. Either that or in a totally different location (like Loch Modan or something) that has the same sort of columns. I will figure this out! Hehe.