Hundreds of pages of secret documents leaked to the ABC provide a rare glimpse into how human censors and AI erase references to Beijing's violent crackdown on pro-democracy protesters in 1989.

It has grown faster, smarter and increasingly invisible, quietly erasing the memory of the 1989 Tiananmen Square massacre from public view.

Thirty-six years on, Beijing still has not disclosed the official death toll of the bloody crackdown on a pro-democracy gathering on June 4, when more than 1 million protesters were in the square.

Historians estimate that the People’s Liberation Army (PLA) killed anywhere from 200 to several thousand people that day.