Meteoritic craters usually call to mind highly visible blemishes, like Arizonas Barringer Crater, Vredefort in South Africa and the moons enormous Orientale basin. On the earth, these impact structures are rather a sorry lot, battered as they are by the elements and geological activity. The resulting erosion makes studying the conditions under which the craters formed difficult at best. To that end, an impact structure described today in the journal Nature may offer important new insights. According to the report, researchers have found and mapped a well-preserved crater hiding beneath the floor of the North Sea, where it formed an estimated 65 million years ago.

The subsea depression was discovered quite by accident, during prospecting of the area for oil and gas. Dubbed the Silverpit crater, it lies under 40 meters of water and a few hundred meters of sediments. Three-dimensional imaging, performed by Simon A. Stewart of BP plc in Aberdeen, U.K., and Philip J. Allen of Production Geoscience Ltd., in Banchory, U.K., revealed a crater some 20 kilometers across with a distinctive multi-ring pattern. Impact structures on the earth and moon generally fall into two categories: simple and complex. The larger the crater, the more complex it tends to be. Silverpit is thus unusual in that it is both relatively small and it has a multi-ring structure previously seen only in craters with diameters of at least 250 kilometers. (Exactly what causes rings to form is a matter of debate.) In fact, size differences notwithstanding, Silverpit actually looks more like the Valhalla crater on Jupiters icy moon Callisto than any terrestrial crater.

But the case for Silverpit being an impact-generated feature (as opposed to the result of some geological process) is not closed yet. "The real test that Silverpit was created by an impact will be to look for shock effects in the rocks that form it," writes Canadian geologist John G. Spray of the University of New Brunswick in an accompanying commentary. "Shock-generated features such as unusual microscopic mineral deformations and shatter cones (conical fracture systems formed by shock waves in rock) would be compelling evidence of an impact origin."