Transmission electron microscopy and atomic force microscopy were used to study the origin of a preferred nucleation site on the atomically flat 3C-SiC mesas leading to the formation of tetrahedral hillocks. The hillocks exhibit a "wedding cake" structure consisting of concentric triangular terraces with a step height of 0.25 nm corresponding to the thickness of a single Si-C bilayer. KOH etching revealed the presence of a threading dislocation at the center of the hillock. Its Burgers vector was shown to lie in the basal plane with no component along the surface normal. It is argued that the strain fields around a threading edge dislocation make it a preferred nucleation site. (C) 2012 Elsevier B.V. All rights reserved.