(S (S (NP (BASENP (NNS Thousands)) (PP (IN of) (BASENP (JJ illegal) (NNS workers)))) (VP (MD can) (VP (VP (VB be) (BASENP (NN underground))) (CONJP (COMMA ,) (CC and)) (VP (VB remain) (S (VP (VP (VBG working) (PP (IN for) (BASENP (NNS weeks) (CC and) (NNS months)))) (ADVP (RB continuously)))))))) (PERIOD .))