depth bomb or depth charge noun

A powerful bomb that explodes under water (dropped over or near submarines)

depth psychologist noun

depth psychology noun

The psychology of the unconscious mind

depth sounder noun

An echo-sounder

depth of field

The distance between the nearer and farther planes in an area photographed, seen through a microscope, etc, over which the image is in reasonably sharp focus

depth of focus

The distance between a camera lens and the film at which the image will be clear

in depth

1. Extending far inwards
2. (of defence) consisting of several successive lines
3. Extensive(ly) and thorough(ly) (inˈ-depthˈ adjective see under in¹)

out of one's depth

1. In water where one cannot touch bottom with one's head above the surface, or too deep for one's safety
2. In a situation, etc beyond one's understanding

the depths

The lowest pitch of humiliation and misery