We investigate the crustal structure of the Dangerous Ground (South China Sea) through processing and interpretation of coincident wide-angle reflection and refraction seismic data. Continental crust of Dangerous Ground has been moderately thinned, down to 15 km, so that most of the structures accompanying the early opening of the South China Sea from Cretaceous to Miocene have been preserved. Subbasement reflectors as well as refraction velocities image an interpreted dismantled Mesozoic metamorphic unit in the southernmost section of our study area. A rollover structure indicates that the reflective base of the unit was used as a décollement where low-angle normal faults root and blocks rafted. The metamorphic unit is discontinued in a nearby basin located immediately to the north, where the refraction velocity model shows thinning of the crust from 20 to 15 km, with the presence of a 5-km-high mantle dome. In this deeper basin, mass transport deposits are found lying on a strong amplitude basement reflector interpreted as the footwall of an~15 km offset crustal detachment surface that we link down to the mantle dome. We infer that the detachment reactivated an inherited low-angle contact most probably related to the Yanshanian belt. In map view, the reactivated structure forms a half-graben basin oriented NNE-SSW oblique to the generally accepted direction of extension. This orientation follows the general trend of a granitic belt that spanned the South China margin prior to extension, related to the subduction of the Paleo-Pacific.