First off, this will work for Rogues. Why? Rogues have shadowstep. That's the key that we're using here. This may work for Warriors using charge, or Mages using blink, but I couldn't say. I'm not a Warrior, nor am I a Mage.
Getting under Zul'Drak as a Rogue. Let's begin.
First off, head to the location below.
Next off, look to the location below and notice the Zul'Drak Bat. This is the mob that's used to get under Zul'Drak. Pay attention to the way it pats. You'll notice it stays in one small area. Mostly near the rock texture you see to the right.
Approach the area where the ground and the rock texture come together. Now, watch the Bat as it patrols and eventually, you'll see in-game what you currently see below.
The Bat actually patrols through the ground just so it can stay on the surface of the rock. The next step should be obvious... SHADOWSTEP.
Obviously, you are now under Zul'Drak. A quick look around and you'll see there's a couple potential possibilities.
Enjoy! I just found this today and I've never seen anyone under Zul'Drak.