I dug up my septic tank to drain it. On the opposite side of the tank I found a PVC cap that was buried below surface. A sewer scope guy said that it was a VENT and that it was connected to a trap(but he didn't actually scope that part). I wondered if it was a VENT, why was it buried below surface? I added some more PVC to the VENT to bring it above ground and drilled some holes in it for oxygen. Did I do the correct thing? If there traps inside the house, why does the septic tank need another one? Why do you think it was buried?