The Abyss (or bottomless pit, KJV) is a very mysterious place, mentioned repeatedly in the Bible! It is not the same as hades or geenna, which is usually translated hell, the fiery place where lost SOULS are tormented. The Abyss is a prison like place of containment for demons. The devil also will be there during the 1,000 year of Christ. It might be referred to in 2 Pet. 2:4, but it isn’t clear:
For if God did not spare angels when they sinned, but sent them to hell, putting them into gloomy dungeons to be held for judgment.
Various translations render that place “hell,” which isn’t consistent. The Greek word there is tartaroo and is only found there and nowhere else in the entire Bible. Also, it is uncertain what angels that sinned mentioned in 2 Pet. 2:4 were sent to tartaroo to be held for judgment. Were those angels the same ones who initially rebelled with the devil or other ones, even possibly the ones referred to Gen. 6:2? It is not clear.
The Abyss is not a pleasant place for demons, as implied by the legion of demons in the man from Gerasenes, begging Jesus not to send them into the Abyss (Lk. 8:31). From Rev. 9:1 there is a key to the shaft of the Abyss. When it was opened, smoke ascended from the Abyss (Rev. 9:2). There is also a king over the demons, which are released from the Abyss. His name is Abaddon (Hebrew) or Apollyon (Greek), which means destroyer (Rev. 9:11). Next the mystery increases with the following two indistinct passages:
Now when they have finished their testimony, the beast that comes up from the Abyss will attack them, and overpower and kill them. (Rev 11:7)
The beast, which you saw, once was, now is not, and will come up out of the Abyss and go to his destruction. The inhabitants of the earth whose names have not been written in the book of life from the creation of the world will be astonished when they see the beast, because he once was, now is not, and yet will come. (Rev 17:8)
The beast that comes from the Abyss could refer to a demon or demonic power working in and with the antichrist during his 42 month reign. See also Rev. 13:2. Last but not least is the final mention of Abyss. It is there that we learn a single angel out of heaven with the key to the Abyss, enchains and locks up the devil for 1,000 years:
And I saw an angel coming down out of heaven, having the key to the Abyss and holding in his hand a great chain. He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years. He threw him into the Abyss, and locked and sealed it over him, to keep him from deceiving the nations anymore until the thousand years were ended. After that, he must be set free for a short time. (Rev 20:1-3)
NOTE: Though various mysteries surround the Abyss, it is clear that God, who is love (1 John 4:8), has an unpleasant place for even the non-humans, who rebel against his will. It seems the mysterious Abyss is reserved for some NOW, before the final lake of fire (Rev. 20:10).