Billed as the largest gathering of humanity in recorded history, Maha Kumbh has commenced in Allahabad. Over the next few weeks, pilgrims will jostle to take a dip at Sangam the confluence of two polluted rivers of the country, the Ganga and the Yamuna. Soma Basu reports on its impacts on the Sangam City and villages downstream, while Bharat Lal Seth analyses why the rivers will remain dirty