A 31-kilometer-wide impact crater underneath about a kilometer of the Hiawatha Glacier's ice is the first of its kind to be discovered in northwest Greenland, scientists report. Although its age cannot be conclusively determined at this time, calculations suggest that the impactor that carved out the crater could have been more than a kilometer wide, which would have had substantial environmental consequences in the Northern Hemisphere or even globally, say the authors. Following ice-penetrating radar analysis of the Hiawatha Glacier, and subsequent additional airborne radar sounding, Kurt H. Kjær and colleagues identified a large circular depression under nearly a kilometer of ice. The internal structure of the Hiawatha Glacier featured three ice layers identified by radar. Using dating techniques, the authors inferred that the young ice covering the crater is "well behaved," but that deeper and older ice was debris-rich and heavily disturbed. The authors traveled to the Greenland Ice Sheet and retrieved three sediment samples deposited by a river draining out of the Hiawatha Glacier. In one sample, angular quartz grains with small fluid inclusions were present and showed signs of being shocked by an impact, the authors say. Several of these grains consist of carbonaceous materials and glass that are likely derived from impact melting of mineral grains in the bedrock. Kjær et al. further tested subsamples of the river sediment and found that it contained elevated concentrations of nickel, cobalt, chromium and gold, indicative of a relatively rare iron meteorite. Kjær and colleagues say tentative age constraints on the crater suggest that it formed during the Pleistocene, though this age is preliminary and the site requires further, extensive study.