It is said to be the ultimate forest and it is where celeby is.....

100 floors and teamstarts at lv.1

purity forest cause you star at level 1and only you with no team members and no
items,money and other pokemon are really powerful

Actually You must climb 99 Floors, but yes it's nearly impossible, most people who make it
to the top are very lucky (I have gotten to the top on my ROM, but of course on my actual
GBA Game I can't) But as many people have stated in order to enter the Dungeon you must
have no team members and you will lose ALL of your items & money you are carrying with you
as you enter. (yes it is the dungeon Whiscash referred to as "The Ultimate Dungeon") And
Celebi IS at the top and you DO NOT have to fight it. It will automatically want to join
your rescue team.