Chilling experiments that took place at 'hellish' Unit 731 as it remains one of the darkest secrets in history

Japan's Unit 731 was once one of the darkest kept secrets in history.

The biological and chemical warfare research unit was funded by the Imperial Japanese Army during World War II, operating in Harbin, Manchukuo (occupied China) from 1937 to 1945.

General Shiro Ishii, a Japanese microbiologist and army medical officer, was in charge of the biological warfare program, with his team primarily consisting of some of Japan's best medical practitioners, according to the Warfare History Network.

Unit 731 was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army (Pictures From History/Universal Images Group via Getty Images)

The unit conducted inhumane experiments on thousands of prisoners, which included vivisections without anaesthesia, frostbite tests, plague and cholera injections, and weapon effectiveness trials.

Reports suggest that between 3,000 and 10,000 people, consisting of men, women and children, died in Unit 731’s experiments.

Backed by the Imperial Japanese Army, the goal of the unit was to develop biological and chemical weapons for the nation.

"None of the people around here had any idea what the real purpose of the facility was," researcher Han Xiao said (via The Daily Mail).

"It was the secret of all secrets - trains could only pass with their curtains drawn; the Air Force would shoot down any plane that came too close."

Shiro Ishii was a Japanese microbiologist and head of Unit 731 biological warfare unit of the Imperial Japanese Army (Pictures From History/Universal Images Group via Getty Images)

Historian Sheldon H Harris noted in his book 'Factories of Death': "If Ishii or one of his co-workers wished to do research on the human brain, then they would order the guards to find them a useful sample.

"A prisoner would be taken from his cell. Guards would hold him while another guard would smash the victim's head open with an ax. His brain would be extracted and rushed immediately to the laboratory.

"The body would then be whisked off to the pathologist, and then to the crematorium for the usual disposal."

A former medical assistant at Unit 731 told the New York Times in 1995 what happened when he cut open a live man for the first time.

Prisoners of war were subject to inhumane experiments (Pictures From History/Universal Images Group via Getty Images)

"The fellow knew that it was over for him, and so he didn't struggle when they led him into the room and tied him down," he said.

"But when I picked up the scalpel, that's when he began screaming. I cut him open from the chest to the stomach, and he screamed terribly, and his face was all twisted in agony. He made this unimaginable sound, he was screaming so horribly. But then finally he stopped.

"This was all in a day's work for the surgeons, but it really left an impression on me because it was my first time."

After Japan’s surrender in 1945, the whole operation stopped.

However, the experiments weren't acknowledged by the Japanese government until 2002 when 180 plaintiffs, primarily from China, sought damages of around £55,000 for the suffering caused, The Guardian reported.

In 2018, 3,607 names of Unit 173 members were disclosed in a list, with Professor Katsuyo Nishiyama telling the Mainichi Shimbun Newspaper: "The list is important evidence that supports testimony by those involved. Its discovery will be a major step toward unveiling concealed facts."