( ( ( ( ( ( The ( earth 's ) ) ( magnetic poles ) ) , ) ( ( ( the ( north pole ) ) and ) ( the ( south pole ) ) ) ) , ) ( ( are ( ( the ( two places ) ) ( on ( ( the surface ) ( of ( ( the earth ) ( where ( ( ( the direction ) ( of ( ( the ( earth 's ) ) ( magnetic field ) ) ) ) ( is vertical ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NP (DT The) (NN earth) (POS 's)) (JJ magnetic) (NNS poles)) (, ,) (NP (NP (DT the) (NN north) (NN pole)) (CC and) (NP (DT the) (JJ south) (NN pole))) (, ,)) (VP (VBP are) (NP (NP (DT the) (CD two) (NNS places)) (PP (IN on) (NP (NP (DT the) (NN surface)) (PP (IN of) (NP (NP (DT the) (NN earth)) (SBAR (WHADVP (WRB where)) (S (NP (NP (DT the) (NN direction)) (PP (IN of) (NP (NP (DT the) (NN earth) (POS 's)) (JJ magnetic) (NN field)))) (VP (VBZ is) (ADJP (JJ vertical))))))))))) (. .)))
The earth's magnetic poles, the north pole and the south pole, are the two places on the surface of the earth where the direction of the earth's magnetic field is vertical.
(ROOT (S (NP (NP (DT The) (NN earth) (POS 's)) (JJ magnetic) (NNS poles)) (VP (VBP have) (VP (VBN switched) (NP (NNS places)) (ADVP (RB repeatedly)) (PP (IN in) (NP (DT the) (NN past))))) (. .)))