noun (n)

• a bottomless gulf or pit; any unfathomable (or apparently unfathomable) cavity or chasm or void extending below (often used figuratively)(noun.object)

  Synonym:
  abysm
  

  source: wordnet30
• A bottomless or unfathomed depth, gulf, or chasm; hence, any deep, immeasurable, and, specifically, hell, or the bottomless pit.(noun)

  source: webster1913