The Abyss or "Bottomless Pit"
The Abyss or Bottomless Pit is for fallen angels who are let in and out according to God's pre-determined purposes. These will be judged and sentenced to "Gehenna" which is The Lake of Fire. The abyss is where the demons in Luke chapter 8 did not want Jesus to cast them into. (Called the pit) The abyss is also referenced in the book of Revelation. The demons are let out for a time and purpose. This is also where satan is bound for a thousand years.

During our present day, satan and a great number of demons are allowed to roam throughout the atmosphere and the earth. There are also certain angels who are locked in the “Abyss” or “Bottomless Pit”. These are allowed to go in and out according to the will of God. Summarizing this place, it is worthy to note that it is a holding tank (if you will), for demons until the final judgment. The location of this holding tank is unclear. There is one passage that is interesting however, in the book of Romans when talking about Christ’s decent into the grave; it uses the word abyss, which is the only reference to someone other than demons going into this place. The abyss is also referred to as the “shaft of the abyss” It is possible that this shaft is the gateway to Hades. That would explain the interchangeable words when referring to Christ. When referring to Christ’s descent Hades is always used, with the exception of the word grave and Jesus himself said he would spend three days and three nights in the “heart’ or the “bowels” of the earth. Wherever the locations are right now isn’t that important, ultimately they all end up in the same place forever.