Question: how long ago was the last time earth's magnetic field reversed
[{'answer': '780,000 years ago', 'provenance': [{'wikipedia_id': '2203131', 'title': 'Geomagnetic reversal', 'start_paragraph_id': 2, 'start_character': 161, 'end_paragraph_id': 2, 'end_character': 178, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]