We report on the nanovoid formation inside synthetic silica, viosil, by single femtosecond pulses of 30-100 nJ energy, 800 nm wavelength, and 180 fs duration. It is demonstrated that the void is formed as a result of shock and rarefaction waves at pulse power much lower than the threshold of self-focusing. The shock-compressed region around the nanovoid is demonstrated to have higher chemical reactivity. This was used to reveal the extent of the shock-compressed region by wet etching. Application potential of nanostructuring of dielectrics is discussed.