(S (S (BASENP (NN Search) (CC and) (NN rescue) (NNS operations)) (VP (VBD were) (VP (VP (VBN suspended) (BASENP (NNP Tuesday))) (PP (JJ due) (PP (TO to) (NP (NP (BASENP (JJ high) (NNS levels)) (PP (IN of) (BASENP (NN methane) (NN gas)))) (SBAR (WHNP (WDT which)) (S (VP (VBD made) (S (BASENP (NNS conditions)) (ADJP (JJ unsafe) (PP (IN for) (BASENP (NNS rescuers)))))))))))))) (PERIOD .))