The term 'Depth of ___' can be filled in with two words, 'PLUMB' and 'ABYSS'. PLUMB is a word that has been used since the 16th century to refer to a tool used for measuring the depth of water. It is also used to describe a vertical line or direction. The word 'ABYSS' is derived from the Greek word 'abussos' which means bottomless. It is often used to refer to an extremely deep hole or chasm, or a vast space or void. Together, these two words can be used to describe the extreme depths of something, such as a body of water or a large expanse of space.