Placed in solitary confinement, he read nothing but the works of Mao, Karl Marx and Friedrich Engels - the only books that were available to him - but they only added doubt to his shaken belief in Communism because he found the reality too far removed from the doctrine of Marxism.

"Persecution makes people mature - I became an opponent of Mao through my time in jail," Song said.

"Those should have been the best years of my life," he said in Hong Kong last month after giving a lecture at Chinese University on the mainland's Great Famine. "So I became determined to find out what the Cultural Revolution was all about."

With dogged determination, Song, who became a historian, spent years painstakingly gathering all the historical documents and materials he could find related to the Cultural Revolution, putting them into a database to let everyone have a clearer picture of the tumultuous period.

"If Mao hadn't locked me up, I wouldn't have been so determined in setting up the database," he said with a grin.

But his efforts to compile information on the Cultural Revolution landed him in jail a second time. In 1999, while working for the Dickinson College library in Pennsylvania, Song was arrested during a trip back to China to gather material for his archive. He was charged with stealing "state secrets" - mostly purchases from second-hand book stalls and antique markets.

His arrest sparked an international outcry and he was released after five months following intervention from the US government.

"The main reason they arrested me was to stop this Cultural Revolution database project," Song said. "They thought it was scary that you could dig up the historical truth."

His troubles only made him want to dig deeper into the history of the Communist Party. Song came to realise that the seeds of the Cultural Revolution were sown in previous political movements such as the anti-rightist movement in 1957 and the Great Leap Forward, a radical economic and social campaign, in the late '50s.

The purging of more than 500,000 intellectuals in the anti-rightist movement eliminated independent and dissenting voices, and when disastrous agricultural policies were implemented in the Great Leap Forward, which precipitated the Great Famine between 1958 and 1961, few dared to oppose them.

"The party was always making bigger mistakes to cover smaller mistakes," Song said.

In his mission to get to the bottom of the Cultural Revolution, he set out to build two other databases - one on the anti-rightist movement and another on the Great Famine.

Song's databases, created in collaboration with six other historians in the United States and China, contain 40,000 historical documents. They include party leaders' speeches and writings, party announcements, internal documents, state media commentaries, Red Guard publications and first-hand material such as self-criticisms, confessions, appeals, pleas and suicide notes written by ordinary people.

The databases are highly regarded by China experts because they have brought together a wealth of material from the Mao era, most of which cannot be easily accessed on the mainland, or which was previously scattered among libraries and private collections across the world.

"He has democratised a period of history that the regime would prefer to ignore," said Frank Dikötter, professor of history at the University of Hong Kong and author of . "Their is no memorial, no museum, no remembrance day for the tens of millions of victims of Communism in China, but his database is a monument that will stand for many years to come."

In last month's lecture on the Great Famine, Song revealed many lesser known facts about the era, known euphemistically on the mainland as the "three years of difficult period". Many historians estimate that at least 30 million died during, but few people realised that many did not die of hunger.

During agricultural collectivisation in the 1950s, the state mandated that peasants hand over or sell all of their grain to the state, leaving them little to survive on.

Song estimates that 10 per cent to 20 per cent of those who died during the famine were peasants who committed suicide or were beaten to death by cadres for failing to meet the grain quota or being accused of hiding grain at home.

The oppression of peasants resulted in thousands of revolts between 1953 and 1957, Song said. According to police statistics at the time, many were armed and organised revolts aimed at toppling the Communist regime - in the first half of 1957 alone, there were several hundred incidents involving up to 100,000 people.

"People were hungry, so they rose up," Song said.

The wave of revolts died down after violent crackdowns by the authorities and the execution of rebel leaders. But the Great Famine followed.

Song said many such facts were now little known because the Communist Party had long monopolised the historical narrative, wanting people to forget its less glorious past. His mission was to record an objective version of history so lessons could be learned.

"A nation that has no collective memory has no future," he said.

In a reflective mood, Song said his life was full of "black humour" - starting out as a fan of Mao, he was accused of being a "counter-revolutionary". But in being incarcerated, he turned into a real opponent of the regime.

"I was whole-heartedly supportive of Mao … but in the end, I have become a genuine counter-revolutionary," he laughed.