People use their smartphones at the Nehru Place IT Market in New Delhi. Jio, an ultracheap mobile network service, increased mobile data usage in India more than sixfold between June 2016 and March 2017. (Sanjit Das/Sanjit Das/Bloomberg)

One recent Wednesday afternoon, monsoon rains were lashing office windows in Mumbai. Inside, screens were lighting up with messages announcing the arrival of Cyclone Phyan.

Employees of a start-up called Little Black Book, an online city guide, started panicking. Some went home early after receiving messages on their phones that roads were being closed. Others passed the message on to loved ones in Phyan's path. Jayati Bhola, a 24-year-old writer at Little Black Book, was organizing a charity music show that evening and feared that the warning may put off her guests. She quickly checked the weather online and then sent around a message: "We're still on guys! Rain or Shine."

As it turns out, Cyclone Phyan never came to Mumbai that evening, Sept. 20. In fact, it had already happened — eight years earlier, 1,400 miles away, in Sri Lanka. "That rumor about the cyclone has been going around for years," said Pankaj Jain, founder of SMHoaxSlayer.com, a website that fact-checks circulating rumors on social media in India.

While fake news in the United States is said to have contributed to President Trump's election victory, in India, a nation with 355 million Internet users, false news stories have become a part of everyday life, exacerbating weather crises, increasing violence between castes and religions, and even affecting matters of public health.

"Common sense is extinct," Jain said. "People are ready to believe anything."

Last week, newspapers here carried full-page advertisements by Facebook that explained how to spot false news. Minister Raj­nath Singh, who oversees home affairs, addressed members of the armed border forces in New Delhi, advising them not to believe everything on social media.

Much of India's false news is spread through WhatsApp, a popular messaging app. One message that made the rounds in November, just after the government announced an overhaul of the country's cash, claimed that a newly released 2,000 rupee bank note would contain a GPS tracking nano-chip that could locate bank notes hidden as far as 390 feet underground. Another rumor, about salt shortages last November, prompted a rush on salt in four Indian states. In southern India, a rumor about a measles and rubella vaccine thwarted a government immunization drive.

Many false stories have led to violence. In May, rumors about child abductors in a village triggered several lynchings and the deaths of seven people. In August, rumors about an occult gang chopping off women's braids in northern India spread panic, and a low-caste woman was killed.

Some stories exacerbate India's rising religious and caste tensions. This week, for instance, images purportedly showing attacks against Hindus by "Rohingya Islamic terrorists" in Burma circulated on social media in India, stoking hatred in Hindu-majority India against Muslim Rohingya.

"There was one video with two people being beheaded, and the text was saying these were Indian soldiers being killed in Pakistan. When I found the original video, it was actually taken from footage of a gang war in Brazil," Jain said. "They'll tell you this is fresh, these are images the media is not showing you, if you're a true Indian patriot, you will forward this message."

The rumors have resulted in a small industry of fact-checkers who are setting up websites to debunk myths circulating online. Pratik Sinha is a former software engineer who started Altnews.in, a fact-checking website. "The number of fake news stories is so high that we can't compete on the quantity of fact checks we do," he said. "We focus on quality."

The fact-checkers come from various backgrounds — some are former journalists, others are software geeks, and some are just concerned citizens. Many fake news stories appear to support India's ruling Bharatiya Janata Party and its right-wing Hindu nationalist agenda, said Jency Jacob, managing editor for boomlive.in, a fact-checking website. "If we don't do something, it will be too late," Jacob said. "Political parties would love to use this for their own benefit and we need to intervene."

The scrutiny has led to some triumphs. Ministers have deleted misleading tweets and posts after being fact-checked online; in one instance, a government ministry launched an inquiry after Altnews pointed out that an image it had used in a report to show floodlights on India's border was actually from the Spanish-Moroccan border.

In September 2016, India's richest man, Mukesh Ambani, launched an ultra-cheap mobile network service, Jio. The new network brought millions of Indians online for the first time. Jio's cheap plans increased mobile data use more than sixfold between June 2016 and March 2017, according to Mary Meeker's Internet Trends 2017 report.

India's first-time users are particularly susceptible to rumors doing the rounds on social-media websites, Jacob said. "The U.S.A. is a more mature market. In India, these forwards take on a life of their own," he said, referring to chain messages on social media.

Asavari Sharma, a Mumbai resident, was one of many who posted images of rainy streets on Facebook. She added a caption that read "CYCLONE PHYAN REACHING US: Hope all the good souls on my list are safe."

"Honestly, I never believe in weather news because every time they show, something never really occurs," Sharma said. This time they had shown a "few horrible images projected straight from the satellite. So I had to upload to Facebook."

By noon, rumors about Phyan had reached authorities. Mumbai's Disaster Management Unit tweeted, "As informed by IMD [India Meteorological Department] there is no cyclone warning for Mumbai. Citizens r requested not to spread & trust rumour."

Jain says the rise of false rumors worries him. "Basically, somebody's making money out of all this," he said, saying that clicks on fake news websites are supported by advertisements. "Ultimately, people are being conned."

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