New here and 1st time posting but i found a new way to get underbelly!
You just need to fly from Zul'gurub in direction of Dalaran(same lvl of Underbelly's altitude).
At some point Dalaran will disapear - the all city and u keep flying till you are inside of the city. you will look all the demons and players over you. keep going to the midddle and you will fall in underbelly.