Crossword Genius

Hard to fathom (4)

Ross

I believe the answer is:

deep

'hard to fathom?' is the definition.
(to fathom is to measure a body of water's depth)

This is all the clue.

(Other definitions for deep that I've seen before include "(Of sound) low in pitch" , "Intense -- extreme" , "down a long way" , "Extending far down" , "Heartfelt" .)

I've seen this clue in The Independent.
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