Waves of white foam blanketed Marina Beach in Chennai, India over the weekend (December 2), attracting local children who were seen playing in the froth.
But local reports say the foam is toxic, caused by pollutants in sewage mixing with seawater and frothed up by waves.
Children have been playing and taking selfies in the clouds of white suds on Marina Beach, even though they give off an acrid smell and fishermen have been told not to go into the sea nearby.
Doctors have warned that skin problems could be caused by the foam, which forms every monsoon season but has been particularly bad this year.
Word has not got through to the hundreds of families who throng India's longest urban beach, letting children happily skip in the toxin-filled froth.
Authorities were also on alert for a repeat of a 2017 incident where thousands of fish were killed by pollution that hit beaches around the same time.
Marina Beach has been a centrepiece of Chennai's life for more than a century.
At weekends, tens of thousands fill the once-pristine sands where the pollution is another sign of India's struggle to keep up with its growing economy.
Experts blame heavy rain in recent days that has carried untreated sewage and phosphate down to the sea.