The souvenir counter in Hong Kong’s June 4 Museum is stocked with books and iconic photos of the 1989 Tiananmen Square massacre in Beijing. To visitors hailing from Mainland China, these items are off-limits; the Chinese government bans all forms of media referencing that bloody day.

Yet many of these tourists visit the counter as they exit the museum, loading up on banned literature to take home. During the trip back, each piece of luggage is scanned at checkpoints, and guards can tell which suitcases are crammed with books. So how to smuggle thousands of contraband pages across the border? “It’s 2015,” one museum staffer explains. “We use pre-loaded USB drives.”

Tourists tuck them in a folded pair of trousers or slip them in back pockets. In sneakernet fashion, bytes of normally inaccessible data make their way into a land of extreme censorship, where only state-trained propaganda officers can sanction historical and political information.

The Chinese government doesn’t have a list of banned book titles, per se. Instead, authorities prohibit the media from mentioning certain topics, such as the Tiananmen crackdown, the personal wealth of Chinese Communist Party leadership, and Hong Kong’s pro-democracy movement.

But in Hong Kong, which gained relative autonomy in 1997, the same restrictions don’t apply. Publishers often use their freedom of speech to print texts that the Party leadership would deem too offensive for Mainland readers. Taiwanese publishers across the strait do the same. And the Mkshft publishers too!

The Hong Kong bookshop People’s Recreation Community has mockingly anointed Chairman Mao as the store’s official logo.

And it’s not just books. Since China’s flourishing pirated DVD shops often self-censor and keep out ‘disruptive’ materials, such as the 1995 documentary “The Gate of Heavenly Place”, people instead download films and TV shows in Hong Kong and walk the files over the border. The phenomenon has become more common in the past year since China’s ruling party launched a crusade to block the use of virtual private networks, which is the primary way residents have eluded government censorship.

Still, plenty of Chinese crave the touch of a real book page—the satisfaction of white paper and dark ink, the smell of a new tome, or the crack of a spine. So Mainland tourists also pick up contraband materials in Hong Kong’s specialty bookshops, street-side news stands, and 7-Eleven stores. If they’re caught back home, offending readers can face penalties as light as confiscation and a verbal warning or as harsh as lengthy interrogations and jail time.

People’s Recreation Community is one of the city’s best banned-book sellers. The name’s three initials mimic the acronym of the People’s Republic of China, or PRC. ‘Recreation Community’ is a sarcastic take on the communes established under Chairman Mao, the founder of Communist China.

In sneakernet fashion, bytes and pages of normally inaccessible materials make their way into a land of extreme censorship: Communist China.

Nestled in Hong Kong’s bustling shopping district of Causeway Bay, PRC’s shop windows are lined with ads for Swiss watches and lunch buffet deals. In the middle is a poster of Mao’s face emblazoned on a red backdrop—the store’s logo. With book titles like Mao Zedong and the Red GuardXi Jinping’s Internal Dialogue, and Princelings: Killing the Nation, PRC caters to a specific class of consumer: educated, curious, and most likely secretive about his or her political leanings. Nearly all of the store’s customers are from the Mainland.

Smuggling physical copies into China requires a more deliberate strategy. Wealthier shoppers drive the books across in their private vehicles to skip the border patrol. Other people who take trains or buses brave the lines at mandatory land-crossing checkpoints and hope they don’t get stopped. To avoid a showdown (or worse), each traveler can only take what looks harmless on the scanner’s screen: one or two books every few weeks.

This incremental approach limits the business of bookstores like People’s Recreation Community. But Paul Tang, PRC’s founder, says it doesn’t matter. “Our customers all come back in droves, and we love seeing them each time they’re back in Hong Kong.”

Slowly, volume by volume, books unseen in China trickle across the border. They are consumed voraciously, then passed from one set of hands to the next, organically growing ideas through people’s immediate circles. The effects of this secret trade can already be seen in Hong Kong, where Mainland Chinese are increasingly joining activities like Hong Kong’s annual June 4 candlelight vigil or the now-defunct Umbrella Movement protests. As banned words continue to inspire curious minds, censored ideas slowly seep past China’s borders, a flow of analog information that even the Great Firewall can’t stem.