Here is a look at the hajj, one of the largest mass gatherings in the world, which starts in Saudi Arabia on Wednesday.

Large numbers of Muslims converge once a year on Saudi Arabia to unite in a series of religious rituals and acts of worship as they perform the Hajj, the pilgrimage that is one of the pillars of Islam.

The Hajj, the annual Islamic pilgrimage to Mecca, is a once-in-a-lifetime obligation for Muslims who are physically and financially able to perform it. It’s a deeply spiritual journey — one that pilgrims dream of for years — but it is also physically demanding and emotionally intense.