Hundreds of millions of Hindus gathered this week in what is expected to be the world's largest human gathering, with a staggering number of devotees, tourists, politicians and celebrities taking holy dips at the convergence of two sacred rivers in India . The religious festival, called the Maha Kumbh Mela, occurs every 12 years on the banks of the Ganges and Yamuna rivers in the northern Indian city of Prayagraj.