Chandogya Upanishad is the second of the two major Upanishads. Its central theme is the correspondence between the cosmos and the fire ritual as it is fashioned around the High Chant. "Chāndogya" means the "singer of the chandas," chandas being the hymns of the Sama Veda, and gya means to sing.
When a treasure of gold lays hidden in the ground, people, who do not know, walk over it every day and never find it. Similarly, all beings move through this world and never see the world of brahma. This is because it is hidden by the unreal. Chandogya Upanishad 8.3.2