(S (S (BASENP (JJ Environmental) (NNS scientists)) (VP (VBP say) (SBAR (S (BASENP (PRP they)) (VP (VBP have) (NP (BASENP (JJ concrete) (NN evidence)) (SBAR (IN that) (S (BASENP (DT the) (NN planet)) (VP (VBZ is) (VP (VBG undergoing) (NP (DT the) (QT (DQL ``) (NP (BASENP (JJS largest) (JJ mass) (NN extinction)) (PP (IN in) (BASENP (QP (NN 65) (NN million)) (NNS years)))) (DQR ''))))))))))))) (PERIOD .))