To put the scale of the event in some context, the largest gathering of Muslim pilgrims at Mecca was in 2005 when an estimated 2.5 million Hajji took part. The last Ardh Kumbh, in 2001, attracted between 70 and 100 million Hindus from all over the world.

The religious significance of the event is profound - Hindus believe that by bathing in the waters of the two rivers, they can wash away sin and escape the endless cycle of death and rebirth, ensuring a place in paradise.

The site is believed to be the location of an ancient battle between gods and demons over an elixir of immortality.

But the spiritual, allegorical purity of the Ganges - considered to be the body of a god on Earth - is undermined by the physical reality. It is one of the most polluted water courses on the planet.

There are myriad reasons for this pollution. There are some 29 cities, over 70 towns and countless villages along the banks, all of which discharge a heady mix of raw and treated sewage, agricultural pollutants and industrial chemicals into the great river.

An estimated 1.3 billion litres of sewage is released into its waters daily, with around 6 million tonnes of chemical fertilisers and 9,000 tonnes of pesticides reaching the river every year.

The religious practice of cremating bodies before scattering the ashes in the holy river has also taken its toll, especially as increasingly scarce wood has become unaffordable for the very poor, meaning half-burned bodies are being dumped in the river.

This year, a gathering of Sadhus - Hindu ascetic holy men - have threatened to boycott the mela, saying the river has become too dirty and the authorities must take tougher action to ensure its cleanliness and purity.

Past studies, including one by international group Greenpeace, have shown the water contains some of the most toxic chemicals known to man.

Although the religious assumption that the purity of the river is divinely protected and germs and pollutants cannot survive its water persists, Hindu priests have tried to fight against this belief and educate the faithful about the problem.

India's Hindu political parties have refused to take any action against the pollution.

As a stop-gap solution, local authorities have said they will release relatively clean water from reservoirs and canals into the river above the site of the festival in the days running up the mass-bathing, providing a temporary flush.

But in the long term, the problem looks set to get worse as the population of the Ganges basin, already in excess of 350 million people, looks set to triple within a generation.