This is probably the most unique way I have ever seen a water heater vented at a home inspection.
From the basement, the vent was routed through the concrete block foundation. Very nicely cemented on the interior, (to prevent too much leaking). It then routed up through the dirt and leaves, terminating at about 4 feet above grade.
I believe the reason the leaves have never ignited is that the soil acted as a heat sink and cooled the metal vent. It was recommended that his system be removed and properly vented.