( ( In ( the ( last ( ( 4 million ) years ) ) ) ) ( , ( ( ( the ( Earth 's ) ) ( magnetic ( north ( and ( south poles ) ) ) ) ) ( ( have ( switched ( ( ( places ( ( ( at least ) 11 ) times ) ) , ) ( ( the ( most ( recent flip ) ) ) ( happening ( ( ( about 780,000 ) years ) ago ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (DT the) (JJ last) (QP (CD 4) (CD million)) (NNS years))) (, ,) (NP (NP (DT the) (NN Earth) (POS 's)) (JJ magnetic) (NN north) (CC and) (NN south) (NNS poles)) (VP (VBP have) (VP (VBN switched) (NP (NP (NP (NNS places)) (NP (QP (IN at) (JJS least) (CD 11)) (NNS times))) (, ,) (NP (NP (DT the) (RBS most) (JJ recent) (NN flip)) (VP (VBG happening) (ADVP (NP (QP (RB about) (CD 780,000)) (NNS years)) (RB ago))))))) (. .)))
In the last 4 million years, the Earth's magnetic north and south poles have switched places at least 11 times, the most recent flip happening about 780,000 years ago.
(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))))) (. .)))