The Kumbh Mela festival, the largest religious gathering in the world, has begun in the northern Indian city of Prayagraj.

While the Kumbh Mela festival is ordinarly held every three years, rotating between several holy cities on the banks of these rivers, the Maha Kumbh Mela occurs only once every 12 years. Maha means “great”, and the event attracts the most devotees because it is considered the most auspicious and holy in the cycle.

India's Maha Kumbh Mela, or Great Pitcher Festival, is set to be the world's largest gathering of humanity, with 400 million visitors expected at the six-week event that began this week in the northern city of Prayagraj.