How the Mecca Stampede Happened

With crowds the size of those on the hajj pilgrimage, normality can turn to chaos and horror in a matter of minutes, according to experts who say yesterday's stampede near Mecca followed a grimly familiar pattern. A big part of the problem is "fluid dynamics—except people are the fluid," writes Adam Rogers at Wired, explaining what happened yesterday appears to have been a "turbulent" stampede caused when two enormous crowds of pilgrims coming down different streets met at a bottle-necked intersection and normal crowd dynamics collapsed. As people continued to pour in, the crowd density quickly became lethal. "People don't die because they panic. They panic because they are dying," professor of crowd science Keith Still tells the AP.

Experts say that crowds can turn deadly so easily because unlike some other animals, humans evolved living in small groups and moving in large numbers doesn't come instinctively. "We don't know how to behave in these scenarios," Princeton biologist Iain Couzin tells Wired. "These situations do not allow us to naturally feel that we can understand what's going on." Psychologists tell the New York Times that the nature of the hajj itself may have played a role. The crowd of pilgrims—many of them elderly—came from all over the world and had no shared language or culture, making communication difficult and probably increasing the sense of panic. In other developments:

• The death toll has now reached 717, and Saudi Arabia's King Salman has promised that there will be an investigation "to improve the level of organization and management of movement" of pilgrims, the BBC reports.
• An adviser to a hajj tour operator in the UK says that while the Saudis blame such disasters on "God's will," the real cause is "man's incompetence," reports the Guardian. He says that pilgrims have told him the deadly movement of people happened because the king was meeting dignitaries at his nearby palace, causing the closure of two entrances to the "stoning of Satan" ritual.

(More Mecca stories.)