In Allahabad, “at the confluence of the Yamuna, Ganges and (mythical) Saraswati Rivers, as many as 100 million people will participate over the next month in an ancient Hindu festival known as the Kumbh Mela.” Urbanists are studying the logistics of what they’re calling a “pop-up megacity”, and social scientists are finding that participating in these gatherings really does bring long-term benefits.