Laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0.6--0.85. Here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of 'room dry' quartz rocks at rates faster than in most laboratory experiments, but slower than seismic slip rates. In some cases, the friction coefficient decreases from low-speed values by more than a factor of 3. This extraordinary weakening likely results from the formation of finely comminuted, amorphous, wet material on the sliding surface.