Cave of Being

Hearth Cave is a location in south of route 20. there you will encounter Professor Juniper, she will tell you that the three sinnoh's pixie pokemon (Uxie,Azelf, and Mesprit) are in Unova and there are rumors they are in the cave in the cave. The three pokemon later showed them-self and will go into another place, Pro.Juniper ask you to find them.

Uxie
Uxie can be found in front of the Nacrene Museum, you need to stand in a specific spot in order it to appear.

Azelf
Azelf can be found in route 23, you need to stand in a specific spot in order it to appear.

Mesprit
Mesprit can be found in the top of Celestial Tower, you need to stand in a specific spot in order it to appear.

Trivia

 * The cave interior is probably the combination of the cave in the sinnoh's lakes.