The Abyss in the Bible refers to hell, a place of torment for the wicked who willfully refuse to acknowledge Jesus Christ as their Savior.
a-bis´, (ἡ ἄβυσσος, hē ábussos): In classical Greek the word is always an adjective, and is used (1) literally, “very deep,” “bottomless”; (2) figuratively, “unfathomable,” “boundless.” “Abyss” does not occur in the King James Version but the Revised Version (British and American) so transliterates ἄβυσσος, ábussos in each case. The King James Version renders the Greek by “the deep” in two passages (Luk 8:31; Rom 10:7). In Revelation the King James Version renders by “the bottomless pit” (Rev 9:1, Rev 9:2, Rev 9:11; Rev 11:7; Rev 17:8; Rev 20:1, Rev 20:3). In the Septuagint abussos is the rendering of the Hebrew word תּהום, tehō̄m̌. According to primitive Semitic cosmogony the earth was supposed to rest on a vast body of water which was the source of all springs of water and rivers (Gen 1:2; Deu 8:7; Psa 24:2; Psa 136:6).
This subterranean ocean is sometimes described as “the water under the earth” (Exo 20:4; Deu 5:8). According to Job 41:32 tehō̄m is the home of the leviathan in which he plows his hoary path of foam. The Septuagint never uses abussos as a rendering of שׁאול, she’ōl (= Sheol = Hades) and probably tehōm never meant the “abode of the dead” which was the ordinary meaning of Sheol. In Psa 71:20 tehōm is used figuratively, and denotes “many and sore troubles” through which the psalmist has passed (compare Jon 2:5). But in the New Testament the word abussos means the “abode of demons.” In Luk 8:31 the King James Version renders “into the deep” (Weymouth and The Twentieth Century New Testament = “into the bottomless pit”). The demons do not wish to be sent to their place of punishment before their destined time. Mark simply says “out of the country” (Mar 5:10).
In Rom 10:7 the word is equivalent to Hades, the abode of the dead. In Revelation (where the King James Version renders invariably “the bottomless pit”) abussos denotes the abode of evil spirits, but not the place of final punishment; it is therefore to be distinguished from the “lake of fire and brimstone” where the beast and the false prophet are, and into which the Devil is to be finally cast (Rev 19:20; Rev 20:10). See also ASTRONOMY, III, 7. -ISBE
I would see the references to hell as a pit or abyss as referring to a sensation of falling. God’s blessings of his present includes “tierra firme” (solid ground) under our feet. Hell is referred to in the Bible and in the rebellion of Korah, the earth opened up and swallowed them whole to hell. So hell should be below the surface of the earth. It is literally “in a pit”. But the concept is what happens when someone rejects God, so even this stability we take for granted is now thrust on the person for eternity as part of their punishment. — David Cox