Spiers (1911) states that the vaults under the ziggurat were 4000 BCE; more recent scholarship revises the date forward considerably but imprecisely, and casts doubt on the methodology and conclusions of the original excavations of 1880. See Gibson, McGuire (1992). "Patterns of Occupation at Nippur". The Oriental Institute of The University of Chicago. Retrieved 21 December 2016.
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"Vault". Encyclopædia Britannica. Archived from the original on 2007-05-22. Retrieved 2007-07-18.
