( ( ( Two categories ) ( of waves ) ) ( ( are ( ground ( waves ( and ( surface waves ) ) ) ) ) . ) )
(ROOT (S (NP (NP (CD Two) (NNS categories)) (PP (IN of) (NP (NNS waves)))) (VP (VBP are) (NP (NN ground) (NNS waves) (CC and) (NN surface) (NNS waves))) (. .)))
Two categories of waves are ground waves and surface waves.
(ROOT (S (NP (EX There)) (VP (VBP are) (NP (NP (CD two) (NNS types)) (PP (IN of) (NP (NN surface) (NNS waves))))) (. .)))