Analysis of the prominent seismic disturbance at the 40-km-diameter Mj¿lnir impact structure, based on an extensive seismic reflection database, shows that the observed broad-brimmed bowl-shaped disturbance was formed as a result of the impact of an asteroid or comet during Volgian-Berriasian time (149--141 Ma). Seismic mapping exhibits and visualizes a 850--1400 km3 disturbed volume and analysis of several structural features within the disturbance provides insight into major cratering processes, such as brecciation and excavation, melting, gravitational collapse of the transient crater, and structural uplift. A transient crater of 16 km in diameter and 4.5 km in depth is determined. From transient and final crater dimensions we obtain an estimate of the degree of gravitational collapse of the order of 2.5, considerably larger than the average expected values for typical terrestrial craters. The extensive collapse took place by low-angle d¿collement surfaces at the periphery and by chaotic debris mass flows toward the center. Furthermore, we estimate the Mj¿lnir projectile to have been 0.9--3 km in diameter and that the physical impact released energy in the range of 2.4--53¿1020 J corresponding to an earthquake of magnitude 8.3. The Mj¿lnir impact is not associated with a significant mass extinction. However, dissipation of the energy released during the Mj¿lnir impact was sufficient to have caused several short-term, near-field perturbations, such as large-amplitude tsunami waves, affecting the Barents Sea region and possibly adjacent areas in the Arctic. ¿ 1998 American Geophysical Union