(S (S (BASENP (DT The) (NNS researchers)) (VP (VP (VP (VBD found) (NP (BASENP (NNS levels)) (PP (IN of) (NP (NP (NP (BASENP (NNP DDT)) (ADJP (BASENP (NN 60) (NNS times)) (JJR higher))) (COMMA ,) (NP (ADJP (BASENP (NN 25) (NNS times)) (JJR more)) (NN mercury))) (CONJP (COMMA ,) (CC and)) (NP (ADJP (BASENP (NN 10) (NNS times)) (JJR higher)) (NN hexachlorobenzene)))))) (PP (IN in) (NP (NNS ponds) (SBAR (IN that) (S (BASENP (NNS seabirds)) (VP (VBD frequented))))))) (PP (VBN compared) (PP (TO to) (NP (BASENP (JJ nearby) (NN ponds)) (SBAR (S (NNS birds) (VP (VP (VBD did) (RB not)) (VP (VB use)))))))))) (PERIOD .))