June 4, 1989 is a date seared into the memories of the Chinese students who were there in the center of Beijing. It was the day the tanks rolled into Tiananmen Square, where students had been leading protests for about six weeks, rallying around their "Goddess of Democracy" statue that was built by art students to resemble the Statue of Liberty.

After the killings, the government seemed determined to root out every last dissident. A reporter from ABC News described the scene, saying "This morning, troops were seen severely beating demonstrators in the Forbidden City district near the square, and at least three of them were arrested. The Goddess of Democracy statue has been toppled by Chinese troops. Students leaders have gone into hiding, and there are now fears the army may move against student sanctuaries on university campuses in the northern part of the capital."

That was 30 years ago today. There has never been an official body count, and the subject is taboo. A generation of Chinese has grown into adulthood shielded from the truth, a history that has been wiped clean, as if the massacre never happened. Hao Jian knows that it did. On a recent panel at Harvard, he recalled the aftermath, heading to the hospital to find his missing cousin among the corpses. "There were dozens of corpses on the floor," he said, "but I failed to recognize him among those corpses." He finally did find his cousin, dead. But by then, he saw how the mothers who had railed against the deaths of their children in the square, demanding accountability, how the government answered with persecution. Even the cemeteries were watched for any signs of protest. Hao Jian says when it came to burying his cousin, it was dangerous even to put the names of the dead on the gravestones. "Only after 6 years did I hire a stone mason to write the name on it," he said.

Jian's detailed account of his cousin's death at the Harvard event marked the first time he had spoken publicly about what he witnessed. Here in the U.S. as a visiting professor, he knew he would be returning to China. Historian Rowena He, who moderated the panel and has long taught a course on the Tiananmen massacre, said Hao Jian was brave to speak out about his cousin like this. "We all know that Tiananmen remains a taboo subject in China today," she said, "and people who openly talk about it will run the risk of being punished." And Jian has been punished in the past, jailed as recently as five years ago, back in China, when he gathered a small group to mark the 25th anniversary of the massacre.

"In 2014, when Professor Hao Jian organized a private commemoration event in his apartment in Beijing," He said, "five of them were taken away by the authorities afterwards."

Another outspoken former student dissident on that recent Harvard panel was Wang Dan, who is now an academic. Thirty years ago, he topped the list of the 20 most wanted by Beijing authorities after the crackdown. He spent years in prison, but in a deal brokered with the United States, he was able, along with other student protest leaders, to come to the U.S. Several finished their degrees in Boston. Wang Dan went on to earn a Masters and Ph.D at Harvard.

The Harvard panel moderator, Professor He, is another who is in exile. But she remembers the day back in China, just after the massacre, when she returned to her campus wearing a black arm band as a symbol of mourning. "My teacher came over to me and said that 'If you do not take that off, no one can protect you from now on,'" He said. "So I removed the black arm band, and I tried to hold back my tears. In the days that followed, I tried to argue with my classmates about whether there was a massacre or it was just a made up story by the western governments, since that's what we were told. Very soon, I just shut up."

But shutting up, she says, is not what she wants from her students these days. Many of them are Chinese studying in the U.S., learning about Tiananmen for the first time. "Some told me that they never heard about it, and then some didn't believe at the beginning," He says. "But after taking the course, they changed, and some reject to listen to me, and some just came with enthusiasm, hoping to find out what happened." But she says there are still those who hold fast to what they've been taught back in China. "Over the years," He said, "I have been attacked by Chinese students who call me a national traitor for teaching the course." This is echoed back in her home country, where it's politically expedient for old friends distance themselves. "I was told that my best childhood friend has been spreading around and saying that I had been collaborating with the western governments to get money," He said. "That's why I've become very rich — by selling my country. And that's very hurtful, and I think it hurts my family."

And it's her family back in China that she misses. It pains her that Chinese students in her class, both those who are eager to learn the history and those who deny it, are free to travel to China while she cannot. "Ordinary graduate students and people who came from abroad — they can just come home to visit," He said. "But that's not the case for me." Still, she says she is driven to wage the battle to preserve the memory of what happened. "I think that just like many others of my generation who have experienced that total crackdown, we wanted to prove one thing: That there is something in this world that cannot be crushed by guns and tanks, and that's the human spirit. That's our longing for freedom and democracy."