It’s late evening in the city of Prayagraj and the roadside vendors selling snacks are packing up their wares to shut for the night.

The 'royal bath' is a key part of the event, held every 12 years in the city of Prayagraj in Uttar Pradesh, significant because Hindus believe it confers salvation from the cycle of birth and death, in addition to absolution of sins.

For pilgrims at the largest gathering of humanity, ritual bathing in India's holy rivers includes a key challenge -- finding your family and clothes after the chilly dip.