Descent into Hell!!! Rarrr!...Well, sort of...

[Shelob]

How about the abyss that Gandalf referred to when he rebuked the Lord of the Nazgul at the Gate of Minas Tirith?

I don't have my books with me right now Lindolirian so I can't go review that section to see how relevent this might be but the abyss could conceivably be based on the trench of Tartarus in Greek myth.

Tartarus is the lowest region of the world, as far below earth as earth is from heaven...Tartarus is described as a dank, gloomy pit, surrounded by a wall of bronze, and beyond that a three-fold layer of night....While Hades is the main realm of the dead in Greek mythology, Tartarus also contains a number of characters. In early stories, it is primarily the prison for defeated gods...in later myths Tartarus becomes a place of punishment for sinners. It resembles Hell and is the opposite of Elysium, the afterlife for the blessed. From Here

Like I said, with out my books to refresh my memory of that part this may be wildly off...but it's the first thing I thought of so with luck it's relevant.