The safe zone, so to speak. Light still reaches this area.
Things grow darker, but light can still be seen.
When in the upper portion there is still light available, the further down the less light available. Near the end of this zone there is complete darkness as it transitions to the next zone.
All light has now disappeared, nothing can be seen except for the faint light of certain see creatures. Blackness is all around, truly and abyss.
Few places reach this depth, it can only be found in certain places in which there are trenches. The darkness continues and what lies ahead is completely unknown.
The deepest point one can reach...what lies beneath must only be creature of unspeakable horror.