The scientists of Unit 731 called their subjects “matures.” The term translates loosely to “logs” — a telling indication of how they were treated, as pieces of wood rather than living creatures.

Though now called the Asian Auschwitz, Unit 731 was originally known as the “Epidemic and Water Purification Unit,” a deceptively benign name. It was home to the some of the most horrific cases of human experimentation ever recorded.

The sprawling complex, nearly four square-miles in area, is thought to be responsible for roughly 200,000 deaths. Korean and Chinese nationals form the vast majority of the deceased, but the complex in the city of Harbin, now part of Northeast China, also held its fair share of Pacific Islanders, Russians, South East Asians, and Allied POWs.

Unlike its notorious European counterpart, the prisoners of Unit 731 were not held out of prejudice but out of convenience, to facilitate the world’s largest biological warfare program. They were human lab rats, and they died from causes as diverse as germ warfare tests, amputation, explosive weapons testing, and vivisection.

Those inside the compound often died of vivisection. One of the medical researchers involved at Unit 731 explained to Take a Moment the process of vivisection conducted on men, women and children, usually without anesthetic:

I was ordered to wash that person’s body with a deck brush before he or she was taken into the dissection room naked by a member of the special team. The first time, I trembled. One team member was listening to the heartbeat with a stethoscope. One was standing holding a knife. The moment the stethoscope was removed from the ear, a knife went into the body. I did not know, but according to doctors, this timing was very important, because if the timing was wrong, we could get blood all over us, and then we could get infected.

Japanese officials wanted to find out the best way to treat shrapnel wounds in wounded soldiers. This desire to keep their soldiers alive and fighting resulted in some of the most violent deaths of Unit 731. Prisoners were tied to wooden stakes beside a bomb placed at various distances. Researchers detonated the bomb and conducted surgery on those who survived, or else proceeded straight to autopsy.

The prisoners were often used as human targets — for flamethrowers and chemical weapons as well as bombs. They had organs removed while they were still alive, even awake, so experimenters could observe them before decomposition altered their state.

Some prisoners had their arms severed to study the effects of blood loss. Sometimes the limbs were reattached or put on ice and thawed to observe the rot and gangrene that set in. Other victims were locked in gas chambers filled with chemicals weapons, or placed in giant centrifuges that spun until those inside perished. Scientists hung some upside down until they choked to death. They injected air into their arteries, or horse urine into their kidneys.

Japan’s widespread use of germ warfare began inside Unit 731. The experimenters had the deadly pathogens at their fingertips, but they sought new ways to deliver the pathogens to cause widespread death.

They were successful on several accounts, developing the defoliation bacilli bomb and the flea bomb. These bombs, often made of porcelain, held fleas carrying bubonic plague, anthrax, typhoid, and dysentery. Fleas were bred by the container-full.

The bombs were dropped on parts of China not yet occupied by Japan to contaminate crops and water supplies. Other times, diseases were spread by giving poisoned candies to children. Japanese scientists wearing hazmat suits might come to the infected towns to inspect the dead bodies to test their work and improve upon it in the future.

BBC estimates these biological weapons killed more than 300,000 confused civilians between 1938-1945.

Despite the devastation inside the walls of Unit 731, scientists never ran short of victims.

“There were always 2,000 or 3,000 logs [people] prepared. There were two burning places and there were always burning dead bodies,” said Shoichi Matsumoto, a Unit 731 bomber pilot.

The deaths at Unit 731 and from the resulting biological weapons never received the same press the crimes of Auschwitz and its “angel of death” Josef Mengele did. There were no Nuremberg Trials, and the perpetrators of the experiments were never brought to justice. Many of them, including Unit 731’s commander and sadist mastermind Shiro Ishii, were honored for service to their country.

Once the Japanese surrendered in 1945, the US government dominated Japanese affairs and discovered the extent of their human experiments. The adversarial dynamics of the cold were already setting in at that point. Officials understood that the findings were valuable, however horrific, since they could never conduct such experiments themselves, and must be kept from the Russians at all costs.

The US officials offered the Japanese scientists responsible a deal: immunity from prosecution in exchange for all experimental data. US General Douglas MacArthur wrote President Truman in 1947, saying “additional data, possibly some statements from Ishii probably can be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as ‘War Crimes’ evidence.”

Despite US efforts to keep the data collected from Unit 731’s singularly sadistic experiments from the Russians, Soviet officials learned of the research during their prosecution of 12 Unit 731 leaders. Those men were sentenced to time in a labor camp, and the USSR used their research to construct a biological research facility of their own in Sverdlovsk.

The 12 officials sentenced in Russia were exceptions. Some of the doctors of Unit 731 rose to prominence in Japan’s post-war medical world, abetted by US occupying forces. The Japanese government has never fully acknowledged the atrocities committed in their nation’s name.