Murray Cox had been gathering data on Airbnb for over a year when he had his first breakthrough. It was December 2015, and the company, in an effort to be more transparent, had just released a trove of data on its presence in New York City. But to Cox, a Brooklyn community activist with a background in product development, something about the data dump smelled fishy. At first glance, Airbnb’s data painted a pretty good picture of the company. It showed, for example, that the vast majority of hosts were listing only one apartment — seeming to debunk the city’s fears of rampant illegal hotel operations. But Cox knew more about Airbnb’s presence in New York City than virtually anyone else outside the company.

In his spare time, the documentary photographer had been scraping information on Airbnb listings across the city and displaying them in interactive maps on his website, InsideAirbnb.com. A self-described “data activist,” Cox updated his numbers regularly—and as he compared Airbnb’s data to his own, he noticed a serious discrepancy. A large chunk of listings seemed to have gone missing just before Airbnb released its report.

Meanwhile in Waterloo, Ontario, a product manager named Tom Slee had noticed the same thing. Since 2013, Slee, like Cox, had been scraping vast amounts of Airbnb data and publishing it to his personal website, where he also ran blog posts denouncing Uber’s presence in Canada and criticizing the broader sharing economy. Given their shared interest, Slee and Cox had known each other for some time, and so they decided to compare notes. The numbers matched up: Both sets of data showed that about a thousand listings had disappeared from Airbnb’s site immediately before the company made its data on some 36,000 listings publicly available.

It quickly became apparent to them that those missing listings represented Airbnb hosts who were placing multiple apartments on the site, in clear violation of a 2010 New York state law. This, they realized, could be huge.

Both Cox and Slee are vocal critics of the sharing economy, and of Airbnb in particular. They charge that it is threatening affordable housing, exacerbating gentrification, and only paying lip service to the ideals of home sharing and community. As Airbnb becomes more powerful and more deeply entrenched in cities, that concern is only growing. One housing advocacy group found that in 2015, Airbnb had reduced the availability of housing in New York City by 10 percent.

A tech company with a $30 billion valuation may seem like a most formidable opponent. But as campaigns across the industry have shown, even just a handful of passionate individuals can bring about change. Look no further than Pinterest engineer Tracy Chou’s demand for data on gender diversity in tech, which triggered a movement, or the first guy who tweeted #DeleteUber, kicking off last month’s frenzy. Small-scale activism, when done cleverly, has an impact — as Cox and Slee soon learned for themselves.

In discovering those missing listings, the data activists realized that their data scraping had just netted them exactly what they had been waiting for: apparent proof that Airbnb was burying evidence of scofflaws on the platform.

“I knew it was big,” says Cox. “We’d basically caught them in as close to a lie as you can get.”

Cox and Slee have never met in person, but through a series of phone calls and emails over the next month, they began to piece together a report.

Per New York’s multiple dwelling law, an apartment in a building with three or more units cannot be rented out for under 30 days unless there’s a permanent occupant present. A host who lists multiple apartments can’t reside in all of them, and so is almost certainly violating the law. The same goes for short-term listings of entire apartments: unless it’s in one of the comparatively rare small buildings in New York, it’s illegal. Yet as others trawling through Airbnb’s data dump quickly observed, more than half the listings in the trove were for entire apartments—and that didn’t even include those vanished thousand-plus listings that Cox and Slee had discovered.

On February 10, 2016, Inside Airbnb published its report with the take-no-prisoners title, “How Airbnb’s Data Hid the Facts in New York City.” In the press release accompanying the report, Cox and Slee wrote that “the intervention was so specific, and the timing so close to the date of the New York City snapshot, that the conclusion is inescapable: Airbnb removed listings from its site so that its data set would paint a more attractive picture of its business, to better influence media and public opinion.”

Almost immediately, the New York Daily News was out with a story on the report; from there, it quickly made the rounds among the New York City media and tech press, and it wasn’t long before it caught the attention of The Guardian and the New York Times. Cox had been right: it was big.

Yet as carefully as the two had pored over the data before publishing, they were worried. At first Airbnb seemed to dismiss their claims, telling the press that the drop in listings was likely a natural fluctuation—a comedown off of the frenzy of the New York City marathon and Halloween weekend, maybe. “You do get a bit nervous,” says Slee. “You put it out and you think, suppose we just missed something? We’d all look like idiots.”

Then, a couple of weeks later, Airbnb sent a letter to New York state legislators and users in which it essentially admitted to the report’s allegations. In the letter, Airbnb’s New York public policy lead, Josh Meltzer, wrote that “last November, we removed approximately 1,500 listings from our platform in New York City that were controlled by commercial operators and did not reflect Airbnb’s vision for our community.” Airbnb’s framing, of course, was that it had been acting in the public interest by removing those commercial operators—but would the company have admitted to the purge if it hadn’t made headlines? The evidence suggests not.

In the year that followed, Airbnb faced increasing pushback in New York. The company’s attempt to curry favor by releasing its own data in 2015 appeared to have done the opposite: Within weeks of the initial data dump, state legislators introduced a bill that would impose heavy fines on anyone advertising an illegal listing on the site. That bill was passed in June 2016 and signed into law in October — and Cox believes that Inside Airbnb’s analysis played no small part in making that happen.

“It would have been very present on electeds’ minds that Airbnb can’t be trusted, at least based on that incident,” he says. The state attorney general’s response to Inside Airbnb’s reporting suggested as much—as a spokesperson told Recode, “Just as it did in wiping 2,000 illegal listings after we confronted the company in 2014, Airbnb once again appears to have manipulated data to conceal illegal activity.”

A partial snapshot of Airbnb listings in New York, as presented on Inside Airbnb.

Airbnb, meanwhile, learned its lesson. The company now regularly removes hosts with multiple listings as part of its “One Host, One Home” policy in New York City. In a statement, a spokesperson told Backchannel that “while scraped data is flawed and presents an inaccurate picture of our community, we’ve committed to provide detailed, accurate information about Airbnb hosts and guests. We haven’t been perfect and we’ve worked diligently to release more information about our community. In New York, we’ve released detailed information about our listings and the number of listings we’ve removed every month.”

After years of operating under a veil of secrecy, Airbnb was coming out into the light.

That first Inside Airbnb report was a watershed moment for Cox’s project, propelling it into the spotlight and emboldening him to release more reports that in turn led to more negative press coverage of Airbnb. One such report examined the risk Airbnb poses to affordable housing in Los Angeles, while another showed that, as of November, few NYC hosts had been deterred from illegally listing their entire apartments, despite the steep new financial penalties. Today, his data shows, illegal short-term listings still proliferate on the site.

In New York, Inside Airbnb’s role has now shifted from that of an investigator to that of a watchdog — but the site has staying power. Indeed, Inside Airbnb’s SEO is so good that a Google search for “Airbnb data” brings up several links to Cox’s site before Airbnb’s own.

Airbnb has said it will work with the city to address host compliance with the new law, and Cox will be watching closely as that plays out. Meanwhile in other cities, regulatory battles are just heating up, and Cox is actively working with tenants rights groups and housing advocates from Toronto to Amsterdam to help them understand Airbnb’s impact on their cities and arm them with the data they need to make their case.

As for how long he’ll keep it going, taxing though the project may be? “Ultimately I think I’d like to stop when Airbnb is appropriately regulated,” says Cox — and he believes that day will come. “Cities are not going to sell out that easily. In the major cities where large-scale tourism in residential neighborhoods is not appropriate, I think the cities will fight back.”