'Pure faith'
A humongous tent city, two-thirds the area of Manhattan, is being built on the floodplains of Prayagraj, formerly called Allahabad, in the northern state of Uttar Pradesh.
"Some 350 to 400 million devotees are going to visit the mela, so you can imagine the scale of preparations," said Vivek Chaturvedi, the spokesman for the festival.
Preparing for the Kumbh is like setting up a new country, requiring roads, lighting, housing and sewerage.
"What makes this event unique is its magnitude and the fact that no invitations are sent to anybody... Everyone comes on their own, driven by pure faith," Chaturvedi told AFP.
"Nowhere in the world will you see a gathering of this size, not even one-tenth of it."
Around 1.8 million Muslims take part in the annual hajj pilgrimage to Mecca in Saudi Arabia.
The Kumbh numbers, according to Chaturvedi, are mind-boggling.
Some 150,000 toilets have been built, 68,000 LED lighting poles have been erected, and community kitchens can feed up to 50,000 people at the same time.
Alongside religious preparations, Prayagraj has undergone a major infrastructure overhaul, and huge posters of Prime Minister Narendra Modi and state Chief Minister Yogi Adityanath dot the city.
Both are from the ruling Hindu-nationalist Bharatiya Janata Party (BJP), with politics and religion deeply intertwined.
Nectar of Immortality
The Kumbh Mela is an ancient celebration, with its origins rooted in Hindu mythology.
Hindus believe that taking a dip in Sangam, the confluence of the rivers, will cleanse them of their sins and help them attain "moksha", setting them free from the cycle of birth and death.
According to legends, deities and demons fought over a pitcher -- or "kumbh" -- containing the nectar of immortality.
During the battle, four drops fell to Earth.
One drop landed in Prayagraj.
The others fell at Haridwar, Nashik and Ujjain -- the three other cities where the rotating Kumbh Mela is held on other years.
But the one in Prayagraj -- held every 12 years -- is the largest.
Organising authorities are calling it the great, or "Maha" Kumbh Mela.
The last Kumbh Mela at Prayagraj in 2019 saw 240 million devotees, according to authorities -- but that was the smaller "Ardh" or half festival, spaced in between the main event.
"When you talk about the Kumbh, you have to talk about astronomy," said historian Heramb Chaturvedi, 69.
"Jupiter transits one zodiac sign in a single year," he added. "Therefore, when it completes 12 zodiac signs, then it is Kumbh."
Core to celebrations is giving alms to the "wise and learned, the poor and the needy", he said.
Naked monks
Some pilgrims have already arrived, including naked naga sadhus -- wandering monks who have walked for weeks from the remote mountains and forests where they are usually devoted to meditation.
They will lead the dawn charge into the chilly river waters on the six most auspicious bathing dates, starting with the first on January 13.
"I have come here to give my blessings to the public," 90-year-old naga sadhu Digambar Ramesh Giri, naked with dread-locked hair in a bun, told AFP.
"Whatever you long for in your heart you get at Kumbh."