Earlier this week, thousands of Hindu men and women observed the Chhath Puja, an ancient festival dedicated to the sun god.

For four days, devotees from northern India and even Nepal fasted, made offerings of sweets and fruits, and immersed themselves in sacred rivers—particularly the Yamuna.

In fact, it’s the festivities around the Yamuna that were the most striking.

One of India’s most sacred rivers, it has turned into one of its most polluted as institutional neglect has allowed industrial effluents and untreated sewage to flow into it over the years. Environmentalists have now labelled it as “ecologically dead.” And though successive state governments have promoted plans to clean up the river, none have delivered as yet.

But for millions, the Yamuna remains a key water source and deeply resonates with their faith. So, for the Chhath Puja, thousands entered its waters covered in toxic foam.

Here are some images of this year’s Chhath Puja, celebrated in the Yamuna river:

Reuters/Anushree Fadnavis A Hindu woman worships the sun god in the polluted waters of River Yamuna during Chhath Puja in New Delhi, on Nov. 14.

Reuters/Anushree Fadnavis Industrial effluents and untreated sewage have polluted the Yamuna. Here, a devotee worships in the sacred river during Chhath Puja in New Delhi, on Nov. 14.

Reuters/Anushree Fadnavis A woman takes part in the Chhath Puja rituals in the Yamuna in New Delhi, on Nov 14. Environmentalists have described the polluted river as “ecologically dead.”

Reuters/Anushree Fadnavis Hindus observe Chhath Puja on the banks of the Yamuna in New Delhi on Nov. 13. Rituals include making offerings of sweets and fruits, and immersing in the sacred river.

AP Photo/Rajesh Kumar Singh Worshipers in the knee-deep waters of the Yamuna, during Chhath Puja in Allahabad, Uttar Pradesh, on Nov. 13.

AP Photo/Altaf Qadri Devotees mark Chhath Puja by making offerings to the sun god in the river Yamuna, in New Delhi, on Nov. 13.