  We argue that the word ``critical'' in the title is not purely literary.
Based on our and other previous work on nonlinear complex dynamical systems, we
summarize present evidence, on the Oct. 1929, Oct. 1987, Oct. 1987 Hong-Kong,
Aug. 1998 global market events and on the 1985 Forex event, for the hypothesis
advanced four years ago that stock market crashes are caused by the slow
buildup of long-range correlations between traders leading to a collapse of the
stock market in one critical instant. We qualify the log-periodic oscillations
using a novel non-parametric method that does not rely on any fit: the
corresponding log-periodogram exhibits a strong statistically significant peak
for all six crashes examined, pointing at approximately the same prefered
scaling ratio around 2.
