Bathymetry is the specific study of which feature of seas and oceans?

I only knew that a bathysphere is used to take people down to deep depths and used that to "fathom" out the answer.

A bathyscaphe (deep ship) called "Trieste" was the first manned vehicle to descend to the bottom of the deepest part of the ocean, the Challenger Deeps in the Marianas Trench in January 1960.