278 IMPACT STRUCTURES impact structures in the terrestrial geological environment They are also areas where there have been active programmes to search for and study impact structures Erosion and sedimentation result in characteristics of the terrestrial impact record that are not as dominant on other planetary bodies For example, approximately one-third of known impact structures are buried by post-impact sediments Most buried impact structures were detected initially as geophysical anomalies and later drilled, for economic or scientific purposes, which confirmed their impact origin Although a few submerged impact structures are known, they occur on the relatively shallow, continental shelves No impact structures are known from the true ocean floors This reflects the relatively young age and the generally poor resolution of geological knowledge of the ocean floors There are also biases in the ages and sizes of known terrestrial impact structures The majority are