Complex sentence: Some think that the Encyclopedia in fact disappeared at the death of Jiajing, having being taken by the emperor to his grave, and they believe that it will yet be found hidden in the tomb complex of Yongling.
Split into simpler sentences: Some think that it in fact disappeared at the death of Jiajing. They believe that the Encyclopaedia was buried with the emperor, and that it will yet be found buried in the tomb complex of Yongling.