Chandogya Upanishad makes a quick metaphorical reference to the universe in chapter 3, section 15. Here is text 1, the opening statement:

antarikṣodaraḥ kośo bhūmibudhno na jīryati
diśo hyasya sraktayo dyaurasyottaraṃ bilaṃ
sa eṣa kośo vasudhānas tasmin viśvam idaṃ śritam

antarikṣa is a common cosmology term. It literally means “inner space” and refers to the space “inside” the human area, opposed to the space outside it. In modern terms it is similar to the “solar system” are opposed to “interstellar space.”

kośa is a container.

bhūmi-budhna – means “foundation”. It is sometimes misrepresented as “bottom (budhna) of the earth (bhūmi).” The meaning “foundation” results from combining bhūmi (ground, base) and budhna (lowest, deepest).

These four words together, in nominative case, give the subject of the sentence: “the foundation (bhūmi-budhna) of the the cosmos (antarikṣa) we dwell in (kośa).”

na jīryati is the verb for this sentence. It means “get old” (from झॄ). Now the whole sentence reads:

“The foundation of our cosmos is eternal.”

diśo hyasya sraktayo dyaurasyottaraṃ bilaṃ

Diśaḥ hi asya sraktayaḥ – “its (asya) corners/edges/bounds (srakta) are the very directions themselves (diśa hi).” The idea is that it is not only eternal – chronologically infinite – it is also boundless – spatially infinite.

Dyas urasya uttaraṁ bilaṁ – “the heavens (dyas) are the ultimate (uttara) peak (uras) and nadir (bila).” This is misrepresented sometimes as “heaven (dyas) is the hole (bila) at the top (uttara) of its breast (uras).”

There is a beautiful poetic connotation. The universe is implied to be female. Her “bottom is wide” (budhna bhūmi), her “hips are broad” (diśa srakta), and her “breasts are high as heaven” (dyasurasya uttaram bilam).

---

sa eṣa kośo vasudhānas tasmin viśvam idaṃ śritam

sa eṣa kośaḥ – “this container” (the container is antarikṣa, our local cosmos)

vasudhānaḥ – “treasure”.

tasmin – “therein”.

viśvam idam – “everything there is” / “the whole world”

śritam – “is placed” / “is situated”

The complete sentence is “In the treasures within this chest rests everything in the entire world.”

The complete statement, Chandogya Upandishad 3.15.1, thus reads:

“The foundation of our cosmos is eternal.
Its boundary is only the directions themselves.
The heavens are both its upper and lower arc.
In this space, every treasure of the entire world exists.”

– Chāndogyopniṣad 3.15.1