  Each year, crowd disasters happen in different areas of the world. How and
why do such disasters happen? Are the fatalities caused by relentless behavior
of people or a psychological state of panic that makes the crowd 'go mad'? Or
are they a tragic consequence of a breakdown of coordination? These and other
questions are addressed, based on a qualitative analysis of publicly available
videos and materials, which document the planning and organization of the Love
Parade in Duisburg, Germany, and the crowd disaster on July 24, 2010. Our
analysis reveals a number of misunderstandings that have widely spread. We also
provide a new perspective on concepts such as 'intentional pushing', 'mass
panic', 'stampede', and 'crowd crushs'. The focus of our analysis is on the
contributing causal factors and their mutual interdependencies, not on legal
issues or the judgment of personal or institutional responsibilities. Video
recordings show that, in Duisburg, people stumbled and piled up due to a
'domino effect', resulting from a phenomenon called 'crowd turbulence' or
'crowd quake'. Crowd quakes are a typical reason for crowd disasters, to be
distinguished from crowd disasters resulting from 'panic stampedes' or 'crowd
crushes'. In Duisburg, crowd turbulence was the consequence of amplifying
feedback and cascading effects, which are typical for systemic instabilities.
Accordingly, things can go terribly wrong in spite of no bad intentions from
anyone. Comparing the incident in Duisburg with others, we give recommendations
to help prevent future crowd disasters. In particular, we introduce a new scale
to assess the criticality of conditions in the crowd. This may allow
preventative measures to be taken earlier on. Furthermore, we discuss the
merits and limitations of citizen science for public investigation, considering
that today, almost every event is recorded and reflected in the World Wide Web.
