Hi there. I've searched but I couldn't find this, I'd have thought this would be rather obvious, or well, that at least someone would have found this. I found this as I was mining, and an Obsidium was hiding in an unreachable spot. As I tried to reach it, I accidentally slammed through the world and fell into open area below it, a few seconds of started flying later I smashed into water and fell to my death.
The entrance is located in Mt. Hyjal, however BE WARNED. Only small races like gnomes, humans and co can enter. For example, I was unable to enter that gap with my night elf warrior. Here are the screenshots of the location in Mt Hyjal. (The actual zone is called Gar'gol's Hovel).
This can always be used as a backup area to get underworld, in case the hole you usually go in gets blocked off, whether you use it for underground mining, herbing and so on or for simple exploration, whatever you want.
Edit: Apparently there are thousands of reposts of this. Tried many varieties of searches and couldn't find it. Oh well, lock or delete this I suppose.