The etching of Si(100) surfaces in ultrapure water was studied with a combination of infrared spectroscopy (FTIR) and scanning tunneling microscopy (STM). While the FTIR results show that the initially rough H/Si(100) surface becomes highly homogeneous during etching, a phenomenon generally associated with surface smoothing, STM images reveal that the homogeneity is associated with the formation of well-defined etch hillocks. After many hours of etching, the resulting H-terminated surface is composed of stripes of atomically flat Si(100) terminated by etch hillocks bounded by {111}- and {110}-oriented microfacets. Polarization analysis of the Si-H stretching modes provides strong evidence for uniform dihydride-termination of the flat regions, with the narrow (approximately 25 A) width of these stripes allowing for relaxation of steric strain between neighboring dihydrides. The unusual hill-and-valley etch morphology is attributed to the effects of steric strain on the reactivity of sites on the etched surface.