PRAYAG, India -- The biggest gathering in human history could take place this weekend in India, during a pilgrimage for one of the world's great religions.
Millions have already arrived; it's a sea of humanity on the Ganges River. From every corner of India, and by every conceivable mode of transport, pilgrims are making their way to Prayag, the holiest place in the Hindu world, for the biggest celebration on the Hindu calendar -- the Maha Kumbh Mela -- held only once every 12 years.
Drawn by their shared faith, they come to purify themselves by bathing in the Ganges, the river that nurtured India's 5,000-year-old civilization.
Om Kumar is a wheat farmer from central India, who told us he walked 300 miles to get here. He made the journey, he said, because the water has special power.
For Hindus, the Ganges is a sacred river. They believe that bathing here during the Kumbh Mela will wash away their sins.
The pilgrims have set up camp in a sprawling city of tents, complete with banks, its own police force and traffic jams.
But on Sunday, which Hindus consider the luckiest day for bathing, they expect the numbers will swell to 40 million people -- the size of New York, London and Paris combined.
Leading the crowds are the Sadhus, India's colorful holy men.
One of them is Mahant Ravindranand Sarswati, who helped explain Hindu beliefs, including reincarnation.
"Death's the start of life," he said. "This is true. Understand?"
Most of the pilgrims make do with basic conditions, but for a wealthy few there's a luxury tented camp, where one of the guests is the Maharani Radika Raje Gaekwad, an Indian princess who normally lives in a palace.
"There's something that binds you all together -- that faith in the Ganges, in the Kumbh, in your gods, as many as there are, yeah, we are one," she says.
A ritual that's been celebrated by Hindus for thousands of years -- in a country of stunning contrasts -- still has the power to draw the faithful.
Watch for more reporting on the festival on Monday's "CBS Evening News."