Easton's Bible Dictionary (1897)
Used to denote (1) the grave or the abyss (Rom. 10:7; Luke 8:31); (2) the deepest part of the sea (Ps. 69:15); (3) the chaos mentioned in Gen. 1:2; (4) the bottomless pit, hell (Rev. 9:1, 2; 11:7; 20:13).
Schaff's Bible Dictionary
DEEP, THE, in Luke 8:31 and Rom 10:7, does not refer to the sea, but to the abyss, the place where lost spirits await their final doom. The same word is rendered the "bottomless pit" in Rev 9:1-2, Rev 1:11; Rev 11:7; Rev 20:13.