Twenty-three years ago today, a man stood in front of a line of tanks in China’s Tiananmen Square. It was one day after a massacre took place there, the government’s response to student protests in the spring of 1989.

The story of how the photograph came to be, as told in FRONTLINE’s 2006 film The Tank Man, is dramatic and deeply moving. It involves an ordinary man with shopping bags, observers in a nearby hotel and a film canister placed covertly in a toilet:

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Almost 20 years after this moment, FRONTLINE filmmaker Antony Thomas sat down with university students in China to ask them what they knew about the iconic image of the tank man. Their answers — ranging from whispers about 1989 to the observation that the photo captured a parade — speak volumes about the nature of a republic that places a high value on controlling who sees what:

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Thomas later called his meeting with the students “a very dangerous experiment”:

Indeed, one of my Beijing friends warned me beforehand that the least we could expect was confiscation of our tape. The only way I could handle this was to interview the students on all kinds of innocuous subjects for 20-plus minutes to relax them, and then to produce the photograph. Once we had registered their reaction, I deliberately moved on to another bunch of innocuous questions. This was important, as I felt sure that the minder wouldn’t jump in and try to stop the session if I kept the discussion going into other areas. Immediately the session was over, the minder took me to one side and asked me if I had produced the tank man picture, but by now that incident was just a tiny fragment in the middle of a one-hour discussion, and he decided to let the matter rest.

Today, images of what happened at Tiananmen Square are still blocked on the Internet in China due to what John Palfrey of Harvard’s Berkman Center for Internet and Society has described as “the world’s most sophisticated means of Internet filtering.”

Words like “candle,” “massacre,” “tank” and “never forget” have been banned in Mandarin on social media sites like Sina Weibo, reports The Atlantic. Even stock market-related searches have been temporarily disabled due to a Shanghai Composite Index fall of 64.89 points — that’s 6/4/89, specifically.

For much more on the legacy of Tiananmen Square, including analysis the Internet in China and accounts from those who participated in the protests, visit our website. You can watch The Tank Man in its entirety here.