Letter from Gregory J Williams, Senior Research Aide, to Professor H Urey, School of Science and Engineering, University of California, La Jolla, California Enclosing a copy of a photograph of a pingo in Greenland [not included with this letter] that he came across in Gottfried Wiegand, 'Fossile pingos in Mitteleuropa'. Says that the pingo is 100 meters in diameter and is almost identical to a Moon crater that Professor Urey recently pointed out. Asks which crater it was. 6 September 1967