Hindu holy men could boycott a ceremony at the pinnacle of their religious calendar because the Ganges, the focus of the festival, is too polluted to bathe in. Even the mighty Ganges struggles to soak up the pollution from India's population explosion and industrialisation. The Ardh Kumbh mela is held at the confluence of the sacred Ganges and Yamuna rivers every six years and regularly smashes records as the largest ever gathering of humans.