THE Himalaya and adjacent Tibetan plateau, constituting Earth's largest region of elevated topography and anomalously thick crust, formed as a consequence of Cenozoic collision between India and Asia—itself considered the archetypal continent–continent collision^1–3. Here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling. Our ∼100-km-long profile, acquired in the Tethyan Himalaya, shows a mid-crustal reflection that prob-ably marks the active thrust fault along which the Indian plate is underthrusting southern Tibet; upper-crustal reflections with geo-metries suggestive of large-scale structural imbrication of the upper crust; and Moho reflections from the base of the double-normal-thickness crust underlying the region. These results lend substantial support to the view that crustal thickening beneath southernmost Tibet was accomplished by wholesale underthrusting of Indian continental crust beneath the structurally imbricated upper crust comprising the Tethyan Himalaya.