WHETHER stishovite, the highest-pressure polymorph of SiO2 known from natural samples, transforms to a denser structure at higher pressures has long been of interest. A suggested transition from rutile to the CaCl2 structure driven by a vibrational-mode instability(1) was supported by the observation of a frequency decrease (softening) of a Raman mode with increasing pressure(2). Subsequent X-ray diffraction measurements provided evidence(3) for stability of the CaCl2 phase near 100 GPa. Electronic-structure calculations predict, however, that the transition occurs at much lower pressure, where a shear modulus vanishes and before the Raman mode softens completely(4). Here we use in situ Raman spectroscopy and a new theoretical model to investigate the high-pressure behaviour of stishovite. At 50 GPa, the pressure dependence of the soft B-1g mode abruptly changes and the E(g) mode splits, as predicted for transformation to the CaCl2 structure. Our results demonstrate that any free silica in the deep mantle (below 1,200-1,500 km) will exist in the CaCl2 structure at considerably lower pressures than previously thought(3).