In 1881 the alpine village of Elm, Switzerland, was nearly wiped out by an enormous rockslide that flowed 2 km down the valley. This disaster made geologists and engineers aware that large masses of rock debris may sometimes behave like a fluid with a low internal resistance to deformation. Since 1881 many other instances of fluidized rockslides have been found, both contemporary and prehistoric. The discovery of such rockslide deposits on Mars and the Moon make it clear that neither air nor water play an essential role in the fluidization process, although they may enhance it. The fluid-like flow of large rock debris masses, evenin vacuo, may be explained by the presence of strong acoustic waves or ‘noise’ within the slide mass. Acoustic waves, generated by the shear flow, diffuse though the strongly-scattering rock debris. Because rocks in the slide remain largely in contact during flow, these waves may transmit large pressure fluctuations elastically without simultaneously transporting large amounts of energy. The pressure fluctuations allow the dry rock debris to yield under a differential stress much smaller than the average overburden. The overburden is briefly relieved by any unusually large pressure fluctuation and a local slippage may take place in the debris mass. If such local slippages are frequent enough, the debris may creep forward under an anomalously small mean shear stress. The theoretical prediction, that rock debris may be fluidized by strong acoustic waves, was tested experimentally in sand subjected to strong ultrasonic shaking. Preliminary observations are consistent with the theory. As predicted, the flow is highly non-Newtonian for weak acoustic fields (strain rate is proportional to the eighth power of the stress), but approaches Newtonian flow as the acoustic intensity increases. Rockslides involve so much strain that the acoustic energy produced during the initial fall cannot last long enough to keep the mass fluidized during the last stages of motion. Acoustic fluidization can account for the mobility of large masses of rock debris only if the acoustic energy is regenerated during flow. It is plausible that shear flows may generate large amounts of acoustic energy and thus regenerate the field, but the efficiency of the process is still uncertain. Theoretical studies suggest that regeneration is adequate to acount for the principal features of large rockslides, but more experimental work is required. Despite these uncertainties, fluidization by sound is the only theory that accounts for the major features of the large rockslides orSturzstrom that have occured on earth and the other planets of our solar system. This theory also accounts for the collapse of large impact craters, the formation of central peaks, and the formation of the multiple ring systems that characterize impact basins.