(S (S (BASENP (NNS Astronomers)) (VP (ADVP (RB previously)) (VP (VBD thought) (SBAR (IN that) (S (BASENP (DT the) (NNS rings)) (VP (VBD were) (ADJP (ADJP (ADVP (RB almost) (RB completely)) (JJ flat)) (COMMA ,) (PP (IN with) (NP (BASENP (NNS changes)) (PP (IN in) (NP (BASENP (NN height)) (PP (IN of) (BASENP (RB only) (DT a) (JJ few) (NNS metres)))))))))))))) (PERIOD .))