Millions of Muslims converge on the Saudi Arabian city.
Mecca is a city, but it's much more than that. Every year, millions of Muslims trek to the Saudi Arabian city on their Hajj, or religious pilgrimage. 3.5 million converge there, as they've been doing since the 7th century. According to The Atlantic:
The pilgrims are are also taking part in the Sa'i, traveling back and forth between the mountains of Al-Safa and Al-Marwah, and Ramy al-Jamarat, in which they throw pebbles at three walls in the city of Mina to show their defiance of the Devil. The rituals and locations date back to the 7th Century, but much of the surrounding area has grown and changed radically over the years. For comparison, I've included several historical images of Mecca as well, dating back as far as 1889.
For many more photos, see The Atlantic's lens blog.