New Defender's Study Bible Notes
9:2 smoke of a great furnace. Compare Genesis 19:28. The escaping smoke seems to indicate to earth-dwellers that there are, indeed, literal fires in Hades. Somewhere on earth there is a great shaft (probably more than one—note Numbers 16:33), through which are carried the souls of the unsaved dead, and from which these demons and the accompanying smoke will issue. The location(s), if this is true, are now so covered and hidden that geologists could never locate them—only Christ has the key(s).