Vast crowds of Hindu pilgrims in India bathed in sacred waters as the Kumbh Mela festival opened on Monday, with organisers expecting 400 million people -- the world's largest gathering of humanity -- to assemble over six weeks.India is the world's most populous nation, with 1.4 billion people, and so is used to large crowds.

Millions of Hindu pilgrims are gathering in India to bathe in sacred waters for the six-week-long Kumbh Mela festival -- from the very old to babies, cave-dwelling monks to science teachers.The millennia-old Kumbh Mela,

The Kumbh Mela, a revered Hindu festival, occurs every three years in four sacred Indian locations: Haridwar, Prayagraj, Ujjain, and Nashik. Every 12 years, each site hosts the grand Maha Kumbh. This year's event will be held in Prayagraj.