Kumbh Mela. Currently under way in Allahabad, India, the three-yearly Kumbh Mela festival is the largest gathering of people on the planet, as up to 70 million Hindus converge to wash away their sins where droplets of the nectar of immortality are said to have been spilt when the gods & demons struggled over it. Of perennial interest to foreigners are the hordes of sadhus (often naked, ascetic holy men) who attend, not always without incident. Recently, however, an Australian historian has cast doubt on the supposedly ancient nature of the mass gathering, suggesting that it was largely invented as a way of circumventing British control following the unsuccessful Indian Mutiny of 1857. [reg for final link: mefi / mefi]
Maha Kumbh Mela from space I know it has been covered, but this kinda thing always gives me goosebumps...
Time to wash up for Hindus. Now don't get me wrong, I'm all for religion and such. But if you wash your sins away in this river, you might wind up with something that won't wash off.