Abyss

"... in one chord, deeper then the Abyss, higher than the Firmament, piercing as the light in the            eye of Ilúvatar, the Music ceased."

- Ainulindalë

The Abyss is only mentioned in Tolkiens work once, this is on page 17 of the Silmarillion in the quote above. It is used metaphorically as a proper noun, not much is known about the Abyss for the reason it dates back to the original concepts of Arda. We know that it is the opposite pole as the Firmament which is the highest place from the ground, for that reason it is the lowest point from Arda's surface. The Abyss is also known as Martalmar or Talmar Ambaren which are so called Earthroots.