The spontaneous, etching-induced transformation of an initially flat Si(100) surface to a completely nanofaceted morphology consisting of overlapping pyramidal hillocks has been observed using a combination of morphological and spectroscopic probes and modeled using a fully-atomistic kinetic Monte Carlo (KMC) simulator of Si(100) etching. A novel silicon etchant has been developed that catalyzes the complete chemical transformation of a Si(100) surfaces into H-terminated Si{111} and Si{110} nanofacets. This finding was confirmed by infrared absorption spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The formation of pyramidal hillocks is highly reproducible and occurs on a time scale of several hours, enabling detailed studies of initial hillock formation and subsequent growth. The formation of microfaceted pyramidal hillocks during etching of Si(100) has previously been attributed to local masking on the surface by deposited impurities, etch products or gas bubbles. These mechanisms assume that an adsorbed impurity or gas bubble decorates the apex of every pyramid. Our atomistic simulations uncovered a second mechanism, one that is intrinsic to the etchant and that generates dynamically self-propagating pyramidal structures. Attempts to distinguish between these two mechanisms through rational modifications of the etchant chemistry will be described. For example, the kinetics of pyramid growth were followed spectroscopically, enabling quantitative assessment of the effects of chemical additives. These observations are more than an intellectual curiosity, as the silicon solar cell industry is actively searching for inexpensive, environmentally-friendly means of pyramidally texturing Si(100) surfaces to reduce reflection losses. Conversely, in microfabricated devices, suppression of pyramid formation is critical to high-yield manufacturing processes. An understanding of the hillock formation process may lead to the rational design of better etchants.