Boyd’s Bible Dictionary:
Abyss, or abode, of lost spirits (Luke 8:31; Rom. 10:7). “Bottomless pit,” (Rev. 9:1-2,11; 11:7).
Concise Bible Dictionary:
In Romans 10:7 “the deep” probably refers to the deep sea, for in Deuteronomy 30:13 (from whence the quotation is made) it is “Who shall go over the sea for us?” and the sea is called “the deep” elsewhere, as Isaiah 51:10; Isaiah 63:13. In Luke 8:31 “the deep” refers to the abyss where evil spirits are confined. It is probably the place where the fallen angels are reserved in chains of darkness (2 Pet. 2:4), where the reference is to tartarus, not gehenna. See BOTTOMLESS PIT.