Cave of Being is a cave located on Route 20. There, the player will encounter Professor Juniper, who will inform him/her that Sinnoh's Lake Trio are in Unova and that there are rumors they are in the cave. When the player enters the cave, the trio is indeed inside and they flee; as a result, Juniper asks the player to capture them for the Pokédex.
The interior and exterior of the cave is shaped like a combination of all three lake caverns of Sinnoh.
They say the deepest part of the cave leads to the Sinnoh region.