Sort of study carried out by bathyscaphe? (2,5)
I believe the answer is:
in depth
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for in depth that I've seen before include "Thoroughly and comprehensively" , "Profoundly" , "Swimmer so able to touch bottom" , "Detailed sort of investigation" , "comprehensive" .)