Optical techniques have advanced considerably in recent years to enable processing of surfaces with a resolution less than the wavelength of light. Despite the highly selective nature of light-matter interactions, however, efforts to increase resolution to the scale of single atoms are hampered by rapid and efficient dissipation of the absorbed energy to the surrounding matrix. Here we show that two-photon surface excitation using ultraviolet light provides a method for selectively removing carbon from diamond surfaces. Polished surfaces etched by this method develop ultra-deep subwavelength structures with morphologies dependent on the polarization of the incident laser with respect to the crystal axes. As well as revealing a practical and versatile method for nano-patterning of diamond surfaces, we show that the results comprise mesoscopic evidence for bond scission via a highly localized optical interaction that may lead to the development of new optical approaches for ultra-nanoscale (<10 nm) surface structuring.