( Waves ( ( are ( ( caused mainly ) ( by ( winds ( which ( , ( ( in turn ) ( , ( are ( caused ( by ( ( uneven heating ) ( over ( ( the ( earth 's ) ) surface ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Waves)) (VP (VBP are) (VP (VBN caused) (ADVP (RB mainly)) (PP (IN by) (NP (NP (NNS winds)) (SBAR (WHNP (WDT which)) (S (, ,) (PP (IN in) (NP (NN turn))) (, ,) (VP (VBP are) (VP (VBN caused) (PP (IN by) (NP (NP (JJ uneven) (NN heating)) (PP (IN over) (NP (NP (DT the) (NN earth) (POS 's)) (NN surface))))))))))))) (. .)))
Waves are caused mainly by winds which, in turn, are caused by uneven heating over the earth's surface.
(ROOT (S (NP (NNP Ocean) (NNS waves)) (VP (VBP are) (ADVP (RB mainly)) (VP (VBN caused) (PP (IN by) (NP (NN wind))))) (. .)))