Depth vessel, as in the hadal zone? (4)
I believe the answer is:
dark
'the hadal zone?' is the definition.
The definition and answer can be both locations as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'depth vessel' is the wordplay.
'depth' becomes 'd' (abbreviation in geometry).
'vessel' becomes 'ark' (ark is a kind of vessel).
'd'+'ark'='DARK'
'as in' acts as a link.
Can you help me to learn more?
(Other definitions for dark that I've seen before include "Black, perhaps - night" , "Lacking light" , "Deprived of light" , "Unlit" , "Opposite of light" .)
