Pokemon Tower Underground is a VIP zone after completing the Pokemon tower: maze.
It's stuffed with VERY rare pokemon.
Also a really nice place to EV-Train a pokemon in speed, since 90% of the pokemon you will encounter are going to be a Zubat or a Rattata.
The Pokemon in this zone are lvl 10-20 so if you want to catch the rare ones it's recomended to have a low level pokemon.
Catchable Pokemon Edit
|C = Common||U = Uncommon||R = Rare||D = Donator|
M = Morning , D = Day , N = Night