The West Antarctic rift system is one of the largest zones of continental extension on Earth. However, little is known of its crustal structure owing to the vast ice sheet that dominates the region. We report on new insights gained from a recent broadband seismic experiment. The 25-km-thick crust measured on the southern flank of the Marie Byrd Land dome suggests that the high topography there is partially supported by a low-density mantle, possibly a hotspot, whereas the interior of the rift appears to be underlain by average-density mantle, suggesting that active volcanism is not present beneath the interior of the West Antarctic Ice Sheet. The fact that a crustal thickness of only 21 km was measured in the Bentley subglacial trench suggests that the region has undergone locally extreme extension.