and the size of the North Sea, the expected number of impact craters would be one. Origin The origin of the crater is currently being hotly debated by the Geoscience community with alternate theories of salt withdrawal and pull-apart basin proposed, raising doubts as to Silverpit's categorization as an impact structure. Evidence in favour of impact origin Other mechanisms for producing a crater were considered and rejected by Allen and Stewart when they discovered the crater. Volcanism was excluded because there were no magnetic anomalies in the crater, which would be expected if eruptions had occurred there. Withdrawal of salt deposits below the crater, known to be a mechanism for the formation of some craters, was ruled out because the Triassic and Permian layers of rock beneath the crater appeared to be undisturbed. Another strong indication that an impact had created the crater was the presence of a central peak – something that Stewart & Allen contend is difficult to form except through a meteorite impact. Evidence for alternative interpretations Analysis of regional 2D seismic lines and 3D seismic volumes by John Underhill, a geologist at the University of Edinburgh, led to the counterproposal that withdrawal of Upper Permian (Zechstein Supergroup) salt at depth was in fact a better explanation. Underhill found that all layers of rock down to the Permian (with an age of about 250 million years) are synclinically folded, and that sediments of Tertiary age at the crater onlap its sides and thicken into its axis, suggesting that the salt was moving (a process called halokinesis) while Tertiary sediments were being laid down. In 2007, Underhill continued to present evidence that he argues does not support the impact hypothesis. After analyzing seismic data over a wide region, he proposed that Silverpit was just one of many similar features related to the withdrawal of the Permian-age Zechstein salt. This result was presented at the April 2007 annual meeting of the American Association of Petroleum Geologists Underhill then focused his research attention upon understanding why the salt moves where it does when it does and why the so-called crater took the form that it did. This led him to publish a peer-review article in the journal, Petroleum Geoscience in August 2009 in which he outlined the evidence for an intrusion-related salt withdrawal cause for the feature's formation. In October 2009, an open debate of the notion that "the Silverpit