There are massive events every year that attract so many people, actual temporary cities sprout up to accommodate them. From the religious worship in Mecca to whatever's going on at Burning Man, check out these seas of humanity so expansive they create their own functioning cities, however briefly.
Every year, upward of 50,000 descend on Nevada's Black Rock Desert to "dedicate themselves to the spirit of community, art, self-expression and self-reliance. They depart one week later, leaving no trace." But in the meantime, they build a literal city, Black Rock City.
Inside a semi-circle of concentric streets, there are temples, yoga tents -- even a temporary airport. It's all set up around the Burning Man effigy, which, naturally, eventually gets burned.
Here's the aerial shot of what that all entails:
And here's what it looks like on the ground:
Hajj, The Pilgrimage To Mecca
Every Muslim is supposed to undertake the hajj -- the pilgrimage to Mecca -- at least once. That religious edict means about 3 million Muslims are on the move every year for the journey, more than doubling Mecca's population.
There's no place to put everyone in the city, so a temporary city springs up around the faithful. This video from 2011 gives a good scope of how expansive the tent city outside Mina is, with its own infrastructure:
And check these out:
Kumbh Mela In India
Kumbh Mela a massive Hindu pilgrimage. And we mean massive. About 100 million people massive. The site of the pilgrimage rotates every third year among four locations.
All of those people have to go somewhere. Up pops another huge, temporary city to house them and even things like bridges to carry people over the Ganges.