(I didn't find this)
This looks something like an Old Naxx entrance hidden way behind stratholme...
It's very wierd.
You run to the right of the entrance, jump up between the 2 sides until you hit your head. Mount on a larger size mount and jump over the ledge. Than I used mount+levitate to get to it. Make sure you DON'T jump off the end or you will just end up inside the entrance to strat. I have seen this done using levitate and rocket boots.