(S (S (S (NP (BASENP (DT The) (NN cause)) (PP (IN of) (BASENP (DT the) (NN rock) (NN fall)))) (VP (VBZ is) (ADJP (JJ unknown)))) (CONJP (COMMA ,) (CC but)) (S (BASENP (NNS police)) (VP (VBP have) (VP (VBN said) (SBAR (IN that) (S (NP (EX there)) (VP (VBD had) (VP (VBN been) (NP (BASENP (DT no) (JJ prior) (NNS signs)) (SBAR (IN that) (S (BASENP (DT the) (NN cliff)) (VP (VBD was) (ADJP (JJ unstable)))))))))))))) (PERIOD .))