plumb the depths of

:  a lead weight attached to a line and used to indicate a vertical direction

:  straight down or up :  vertically

:  to a complete degree :  absolutely

:  in a direct manner :  exactly

:  to weight with lead

:  to measure the depth of with a plumb

:  to examine minutely and critically

:  exactly vertical or true

:  thorough, complete

:  a deep place in a body of water

:  a part that is far from the outside or surface

:  abyss