Cave of Being (Japanese: 心の空洞 Cave of Spirit) is a cavern located on Route 20. It is described as being connected to Sinnoh.
Professor Juniper can be found inside here after the player has defeated the Champion. If she is spoken to, Uxie, Mesprit and Azelf will appear. They cannot be caught here; rather each Pokémon will travel to a different part of Unova, awaiting capture. Uxie can be found outside Nacrene Museum, Mesprit will be at Celestial Tower, and Azelf will be on Route 23.
Read more: http://bulbapedia.bulbagarden.net/wiki/Cave_of_Being