  Many observations in the dynamics of pedestrian crowds, including various
self-organization phenomena, have been successfully described by simple
many-particle models. For ethical reasons, however, there is a serious lack of
experimental data regarding crowd panic. Therefore, we have analyzed video
recordings of the crowd disaster in Mina/Makkah during the Hajj in 1426H on
January 12, 2006. They reveal two subsequent, sudden transitions from laminar
to stop-and-go and ``turbulent'' flows, which question many previous simulation
models. While the transition from laminar to stop-and-go flows supports a
recent model of bottleneck flows [D. Helbing et al., Phys. Rev. Lett. 97,
168001 (2006)], the subsequent transition to turbulent flow is not yet well
understood. It is responsible for sudden eruptions of pressure release
comparable to earthquakes, which cause sudden displacements and the falling and
trampling of people. The insights of this study into the reasons for critical
crowd conditions are important for the organization of safer mass events. In
particularly, they allow one to understand where and when crowd accidents tend
to occur. They have also led to organizational changes, which have ensured a
safe Hajj in 1427H.
