By CHRISTOPHER HUDSON

Last updated at 23:50 02 March 2007

After more than 60 years of silence, World War II's most enduring and horrible secret is being nudged into the light of day. One by one the participants, white-haired and mildmannered, line up to tell their dreadful stories before they die.

Akira Makino is a frail widower living near Osaka in Japan. His only unusual habit is to regularly visit an obscure little town in the southern Philippines, where he gives clothes to poor children and has set up war memorials.

Mr Makino was stationed there during the war. What he never told anybody, including his wife, was that during the four months before Japan's defeat in March 1945, he dissected ten Filipino prisoners of war, including two teenage girls. He cut out their livers, kidneys and wombs while they were still alive. Only when he cut open their hearts did they finally perish.

These barbaric acts were, he said this week, "educational", to improve his knowledge of anatomy. "We removed some of the organs and amputated legs and arms. Two of the victims were young women, 18 or 19 years old. I hesitate to say it but we opened up their wombs to show the younger soldiers. They knew very little about women - it was sex education."

Why did he do it? "It was the order of the emperor, and the emperor was a god. I had no choice. If I had disobeyed I would have been killed." But the vivisections were also a revenge on the "enemy" - Filipino tribespeople whom the Japanese suspected of spying for the Americans.

Mr Makino's prisoners seem to have been luckier than some: he anaesthetised them before cutting them up. But the secret government department which organised such experiments in Japanese-occupied China took delight in experimenting on their subjects while they were

still alive.

A jovial old Japanese farmer who in the war had been a medical assistant in a Japanese army unit in China described to a U.S. reporter recently what it was like to dissect a Chinese prisoner who was still alive.

Munching rice cakes, he reminisced: "The fellow knew it was over for him, and so he didn't struggle when they led him into the room and tied him down. 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." The man could not be sedated, added the farmer, because it might have distorted the experiment.

The place where these atrocities occurred was an undercover medical experimentation unit of the Imperial Japanese Army. It was known officially as the Anti-Epidemic Water Supply and Purification Bureau - but all the Japanese who worked there knew it simply as Unit 731.

It had been set up as a biological warfare unit in 1936 by a physician and army officer, Shiro Ishii. A graduate of Kyoto Imperial University, Ishii had been attracted to germ warfare by the 1925 Geneva Protocol banning biological weapons. If they had to be banned under international law, reasoned Ishii, they must be extremely powerful.

Ishii prospered under the patronage of Japan's army minister. He invented a water filter which was used by the army, and allegedly demonstrated its effectiveness to Emperor Hirohito by urinating into it and offering the results to the emperor to drink. Hirohito declined, so Ishii drank it himself.

A swashbuckling womaniser who could afford to frequent Tokyo's upmarket geisha houses, Ishii remained assiduous in promoting the cause of germ warfare. His chance came when the Japanese invaded Manchuria, the region in eastern China closest to Japan, and turned it into a puppet state.

Given a large budget by Tokyo, Ishii razed eight villages to build a huge compound - more than 150 buildings over four square miles - at Pingfan near Harbin, a remote, desolate part of the Manchurian Peninsula.

Complete with an aerodrome, railway line, barracks, dungeons, laboratories, operating rooms, crematoria, cinema, bar and Shinto temple, it rivalled for size the Nazis' infamous death camp of Auschwitz-Birkenau.

The numbers of prisoners were lower. From 1936 to 1942 between 3,000 and 12,000 men, women and children were murdered in Unit 731. But the atrocities committed there were physically worse

than in the Nazi death camps. Their suffering lasted much longer - and not one prisoner survived.

At Unit 731, Ishii made his mission crystal clear. "A doctor's God-given mission is to block and treat disease," he told his staff, "but the work on which we are now to embark is the complete opposite of those principles."

The strategy was to develop biological weapons which would assist the Japanese army's invasion of south-east China, towards Peking.

There were at least seven other units dotted across Japanese-occupied Asia, but they all came under Ishii's command. One studied plagues; another ran a bacteria factory; another conducted experiments in human food and water deprivation, and waterborne typhus.

Another factory back in Japan produced chemical weapons for the army. Typhoid, cholera and dysentery bacteria were farmed for battlefield use.

Most of these facilities were combined at Unit 731 so that Ishii could play with his box of horrors. His word was law. When he wanted a human brain to experiment on, guards grabbed a prisoner and held him down while one of them cleaved open his skull with an axe. The brain was removed and rushed to Ishii's laboratory.

Human beings used for experiments were nicknamed "maruta" or "logs" because the cover story given to the local authorities was that Unit 731 was a lumber mill. Logs were inert matter, a form of plant life, and that was how the Japanese regarded the Chinese "bandits", "criminals" and "suspicious persons" brought in from the surrounding countryside.

Shackled hand and foot, they were fed well and exercised regularly. "Unless you work with a healthy body you can't get results," recalled a member of the Unit.

But the torture inflicted upon them is unimaginable: they were exposed to phosgene gas to discover the effect on their lungs, or given electrical charges which slowly roasted them. Prisoners were decapitated in order for Japanese soldiers to test the sharpness of their swords.

Others had limbs amputated to study blood loss - limbs that were sometimes stitched back on the opposite sides of the body. Other victims had various parts of their brains, lungs or liver removed, or their stomach removed and their oesophagus reattached to their intestines.

Kamada, one of several veterans who felt able to speak out after the death of Emperor Hirohito, remembered extracting the plague-infested organs of a fully conscious "log" with a scalpel.

"I inserted the scalpel directly into the log's neck and opened the chest," he said. "At first there was a terrible scream, but the voice soon fell silent."

Other experiments involved hanging prisoners upside down to discover how long it took for them to choke to death, and injecting air into their arteries to test for the onset of embolisms.

Some appear to have had no medical purpose except the administering of indescribable pain, such as injecting horse urine into prisoners' kidneys.

Those which did have a genuine medical value, such as finding the best treatment for frostbite - a valuable discovery for troops in the bitter Manchurian winters - were achieved by gratuitously cruel means.

On the frozen fields at Pingfan, prisoners were led out with bare arms and drenched with cold water to accelerate the freezing process.

Their arms were then hit with a stick. If they gave off a hard, hollow ring, the freezing process was complete. Separately, naked men and women were subjected to freezing temperatures and then defrosted to study the effects of rotting and gangrene on the flesh.

People were locked into high-pressure chambers until their eyes popped out, or they were put into centrifuges and spun to death like a cat in a washing machine. To study the effects of untreated venereal disease, male and female "logs" were deliberately infected with syphilis.

Ishii demanded a constant intake of prisoners, like a modern-day Count Dracula scouring the countryside for blood. His victims were tied to stakes to find the best range for flame-throwers, or used to test grenades and explosives positioned at different angles and distances. They were used as targets to test chemical weapons; they were bombarded with anthrax.

All of these atrocities had been banned by the Geneva Convention, which Japan signed but did not ratify. By a bitter irony, the Japanese were the first nation to use radiation against a wartime enemy. Years before Hiroshima, Ishii had prisoners' livers exposed to X-rays.

His work at Pingfan was applauded. Emperor Hirohito may not have known about Unit 731, but his family did. Hirohito's younger brother toured the Unit, and noted in his memoirs that he saw films showing mass poison gas experiments on Chinese prisoners.

Japan's prime minister Hideki Tojo, who was executed for war crimes in 1948, personally presented an award to Ishii for his contribution in developing biological weapons. Vast quantities of anthrax and bubonic plague bacteria were stored at Unit 731. Ishii manufactured plague bombs which could spread fatal diseases far and wide. Thousands of white rats were bred as plague carriers, and fleas introduced to feed on them.

Plague fleas were then encased in bombs, with which Japanese troops launched biological attacks on reservoirs, wells and agricultural areas.

Infected clothing and food supplies were also dropped. Villages and whole towns were afflicted with cholera, anthrax and the plague, which between them killed over the years an estimated 400,000 Chinese.

One victim, Huang Yuefeng, aged 28, had no idea that by pulling his dead friend's socks on his feet before burying him he would be contaminated.

All he knew was that the dead were all around him, covered in purple splotches and lying in their own vomit. Yuefeng was lucky: he was removed from a quarantine centre by a friendly doctor and nursed back to health.

But four relatives died. Yuefeng told Time magazine: "I hate the Japanese so much that I cannot live with them under the same sky."

The plague bombing was suspended after the fifth bacterial bombing when the wind changed direction and 1,700 Japanese troops were killed.

Before Japan surrendered, Ishii and army leaders were planning to carry the war to the U.S. They proposed using "balloon bombs" loaded with biological weapons to carry cattle plague and anthrax on the jet stream to the west coast of America.

Another plan was to send a submarine to lie off San Diego and then use a light plane carried on board to launch a kamikaze mission against the city. The war ended before these suicidal attacks could be authorised.

As well as Chinese victims, Russians, Mongolians, Koreans and some prisoners of war from Europe and the U.S. also ended up in the hands of Ishii, though not all at Unit 731.

Major Robert Peaty, of the Royal Army Ordnance Corps, was the senior British officer at Mukden, a prisoner-of-war camp 350 miles from Pingfan. Asked, after the war, what it was like, Peaty replied: "I was reminded of Dante's Inferno - abandon hope, all ye who enter here."

In a secret diary, Peaty recorded the regular injections of infectious diseases, disguised as harmless vaccinations, which were given to them by doctors visiting from Unit 731. His entry for January 30, 1943, records: "Everyone received a 5cc typhoid-paratyphoid A inoculation."

On February 23, his entry read: "Funeral service for 142 dead. 186 have died in 5 days, all Americans." Further "inoculations" followed.

Why, then, after the war, were nearly all the scientists at Unit 731 freed? Why did Dr Josef Mengele, the Nazi 'Angel of Death' at Auschwitz, have to flee to South America and spend the rest of his life in hiding, while Dr Shiro Ishii died at home of throat cancer aged 67 after a prosperous and untroubled life?

The answer is that the Japanese were allowed to erase Unit 731 from the archives by the American government, which wanted Ishii's biological warfare findings for itself.

In the autumn of 1945, General MacArthur granted immunity to members of the Unit in exchange for research data on biological warfare.

After Japan's surrender, Ishii's team fled back across China to the safety of their homeland. Ishii ordered the slaughter of the remaining 150 "logs" in the compound and told every member of the group to "take the secret to the grave", threatening death to anybody who went public.

Vials of potassium cyanide were issued in case anyone was captured. The last of his troops blew up the compound.

From then on, a curtain of secrecy was lowered. Unit 731 was not part of the Tokyo War Crimes Tribunal. One reference to "poisonous serums" being used on the Chinese was allowed to slip by for lack of evidence.

Lawyers for the International Prosecution Section gathered evidence which was sent directly to President Truman. No more was heard of it.

The Americans took the view that all this valuable research data could end up in the hands of the Soviets if they did not act fast. This was, after all, the kind of information that no other nation would have had the ruthlessness to collect.

Thus the Japanese were off the hook. Unlike Germany, which atoned for its war crimes, Japan has been able to deny the evidence of Unit 731. When, as now, it does admit its existence, it refuses Chinese demands for an apology and compensation on the grounds that there is no legal basis for them - since all compensation issues had been settled by a treaty with China in 1972.

Many of the staff at Unit 731 went on to prominent careers. The man who succeeded Ishii as commander of Unit 731, Dr Masaji Kitano, became head of Green Cross, once Japan's largest pharmaceutical company.

Many ordinary Japanese citizens today would like to witness a gesture of atonement by their government. Meanwhile, if they want to know what happened, they can visit the museum that the Chinese government has erected in the only building at Pingfan which was not destroyed.

It does not have the specimens kept at Unit 731: the jars containing feet, heads and internal organs, all neatly labelled; or the six-foot-high glass jar in which the naked body of a Western man, cut vertically in two pieces, was pickled in formaldehyde.

But it does give an idea of what this Asian Auschwitz was like. In the words of its curator: "This is not just a Chinese concern; it is a concern of humanity."