The Abyss is the lower places of the Timeless Halls of Eru.
The opposite of this is the Firmament, the high places of the Timeless Halls.
This is mentioned on page 17 of The Silmarillion.
- "In the midst of this strife, whereat the halls of Ilúvatar shook and a tremor ran out into the silences yet unmoved, Ilúvatar arose a third time, and his face was terrible to behold. Then he raised up both his hands, and in one chord, deeper than the Abyss, higher than the Firmament, piercing as the light of the eye of Ilúvatar, the Music ceased."
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