The deep ethereal is the most remote and secluded part of the Ethereal plane. This section of the plane is generally unmapped and contains many of the most mysterious elements of the plane. The depths of the plane of potential are not a place to go unprepared.
The deep ethereal is not directly reachable from other planes - in order to reach the deep ethereal a traveller must first go through the Border Ethereal. The transistion from border ethereal to deep ethereal is usually marked by an ethereal+curtain. Creatures that are co-terminus and most spells relying on the etherial occur within the border ethereal which overlaps with other planes, not the deep ethereal which does not.
Sources:
The Manual of the Planes (2nd ed.)
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