The fire in Yamlur lake, one of the city's most polluted, has baffled those who live in the area.
"This fire was around for one hour... we also observed it yesterday," said a resident.
Much like the foamy lake, the flames are also the result of severe pollution, say experts.
"There is a floating oil, diesel such things which come from service stations, garages. This oily material also is floating over the river. It has caught fire. How it has caught fire, whether some miscreants have put a fire or it has caught on its own, I'm not very sure," Pollution Control Board chairman Vaman Acharya told NDTV.
"Froth and fire is momentary magic. They're catching the eye. They are more of visual pollution. The real pollution is beneath. We have to be worried about the real pollution that is in the water," he added.
"Effluents rich in hydrocarbons have flowed into the lake from industries surrounding the lake. Both the foam and flames is because of the pollutants," said Professor TV Ramachandra of the Ecological Sciences Department at the Indian Institute of Science.
He says the froth on Varthur lake is also mainly filth - a mix of detergent, urine and faeces - and it has been getting worse every year.
Experts say urgent steps are needed to save Bengaluru's lakes. It has been said by environmentalists, by the High Court, and by the public.
"Foam, fire, what next? A Loch Ness monster?" quipped a resident who lives near Yamlur lake.
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