The biggest human migrations in the world

The hajj, a 5-day pilgrimage to Mecca, Saudi Arabia, began on August 9. It is among the largest gatherings in the world. According to estimates from Saudi Arabia’s Director General of Passports, by Wednesday 1.8 million people had already arrived.

The hajj is a religious ritual for Muslims around the world. All Muslims who are physically and financially capable are expected to make the journey once in their lives. The annual pilgrimage retraces events from the lives of prominent prophets in the Quran, making it one of the five pillars of Islam.

The gathering of millions tests the limits of Saudi Arabia’s transportation and visa system. According to the Saudi Arabian General Authority for Statistics, about 1.6 million people flew into Mecca last year, roughly the size of Mecca itself, which is estimated to be home to 1.9 million people. Nearly doubling the size of the population creates security and crowd control concerns for authorities. Stampedes are a reoccurring problem. In 2015, a stampede killed 700 people.