A rather literal term referring to the action of deep diving in a common household underground storage tank containing fecal matter.
The use of this term is a great term to use to insult someone!
(see below)
Joey - Screw you Curtis, why don't you go for a septic tank deep dive you ass!
Curtis - I have chlamydia.
Milton - What do you do for work Henry?
Henry - I go for septic tank deep dives in peoples front yards!