What is it? I just hit 70 Strength and can access part of it, but what is it?

It's a fairly small dungeon space-wise. There is a main chamber with basically all types of monsters from the dungeon in it. The dungeon has four gods in it: Saradomin, Zamorak, Bandos, Aramdyl. Each god's followers inhabit the dungeon. There is one chamber for each god (and a central chamber with all gods combined). If you wear a piece of clothing of any god, that god's followers won't be aggressive against you. If you kill 40 monsters of certain god during a trip, you will be allowed to enter the boss chamber for that god. The boss is around lvl 600-650 and has 3 guardians in that chamber that are 150-200. You don't have to fight these guys if you don't want to, though; you can just stick to the normal monsters in the dungeon. The dungeon does have a lot of new monsters, though. For more information, read about the dungeon in the Runescape Knowledge Base.