a human volcano or an overflowing septic tank 
(ROOT (NP (NP (DT a) (JJ human) (NN volcano)) (CC or) (NP (DT an) (VBG overflowing) (NN septic) (NN tank))))
(ROOT (NP (NP (DT ) (JJ ) (NN )) (CC ) (NP (DT ) (VBG ) (NN ) (NN ))))