I'm assuming you're talking about the Abyssal Ruins. It contains lots of relic items which are very valuable and you can sell for many thousands of Pokedollars.
There is also most of the "Arceus plates", except two that are already given to you elsewhere. They each raise the power of a type of move by 20% (e.g. Splash Plate raises Water-type moves).
There are 4 different levels to the ruins. I've explored the entire thing and drawn a map which I will post on the site later tonight.