With a thud that shakes the earth beneath our feet, mud, rock and debris are tipped out of the back hoe of the digger, running down the sides of the gathering pile.
What the excavators hope to unearth are human remains that are evidence of atrocities committed by Japanese military doctors in the years immediately before and during the second world war.
On this plot of land, which still bears the marks of a disused car park, a team of seven men supervised by Kazuhiko Kawauchi began digging the first of six large holes last week. The first excavation revealed the crumbling foundations of a building, which, historians claim, housed the Imperial Japanese Army's medical college where an unknown number of people died in tests designed to reveal the possibilities of chemical, biological or bacteriological weapons.
Set up in 1932 by Major Shiro Ishii in Tokyo's Toyama district, its activities eventually grew so large it was transferred to the northern Chinese city of Harbin and named Unit 731.
Major Ishii and his colleagues then roamed across China for 13 years, carrying out experiments that had been planned and prepared in Harbin, says Jing Chen-min, director of the Unit 731 Museum. They dropped ceramic bombs that contained infected insects from aircraft. They deliberately discarded food contaminated with pathogens fully aware that local people would eat it when they left. Pens and walking sticks were reportedly impregnated with viruses that would then be passed on to anyone who picked them up. They detonated germ bombs and tied naked civilians that they referred to as 'murata' - or 'logs' - to stakes in the sub-zero Manchurian winters and sprayed water on them until they were frozen. Frostbite tests were then conducted to see how much force was required to shatter a limb.
But experiments were still going on in Tokyo right up to the dying days of the conflict and as Japan awaited the arrival of the occupying forces, the officers at the medical college ordered the staff to bury the remains of their victims and remain silent.
Kawauchi's team is digging five years after Toyo Ishii broke her silence about what she did in the days after Japan's surrender. A former nurse with the unit, Ishii announced that she had personally buried numerous corpses, bones and body parts to conceal them from the allies.
The site where Kawauchi presently stands is where Ishii was told there was a burial site.
'Ishii was working in this area but was not able to exactly pinpoint the areas where she says remains were buried,' says Kawauchi. 'That is why we are digging six holes and we expect to finish the excavation work in the next two weeks or so.'
He believes there is a 50-50 chance of finding something. 'We're specifically looking for bones and human remains and, if we do find any, we will hand them over to the police,' he said.
Standing behind a gate onto the site, 81-year-old Noboru Watanabe is visibly fretting. A member of The Association Demanding an Investigation into Human Bones Discovered on the Site of the Army Medical College, Watanabe fears that if nothing is found here, the government will refuse to fund a dig at another site that he believes has a far higher chance of yielding human remains. This is the site where nurse Ishii says that she personally buried bodies and is today a park attached to a slightly dilapidated apartment block.
'If they find something where they are digging now, then the pressure will increase on the government to dig here,' he says. 'But if they don't then they will say it is a waste of the Y100 million (HK$9.52 million) the work has cost.'
And that would be a disaster, Watanabe says, because it could spell the end of an investigation into imperial Japan's wartime atrocities. It would also end any discussion of whether Emperor Hirohito knew about what was going on in his name at Unit 731 and whether he approved of its activities.
Watanabe has no doubts the late emperor knew exactly what was being carried out by Major Ishii's units here and in occupied China, South-East Asia and parts of the Pacific. When construction workers were excavating the foundations of the nearby National Institute of Infectious Diseases in 1989, they came across 62 skulls and hundreds of other human bones. Experts finally estimated the partial remains came from around 100 individuals of Asian descent, but were unable to identify them by nationality.
Some of the skulls had bullet holes in their foreheads; others had neatly drilled holes. Some had been cut precisely around the top, apparently to give the doctors access to the brain cavity. Police said they were unable to open a criminal investigation based on evidence that was more than 20 years old. An investigation by the Ministry of Health, Labour and Welfare concluded there was no way to definitively link the bones to Unit 731 and decided the remains were probably bodies used at the college for 'medical education'.
In the grounds of the institute is a tall, stone marker that has been preserved and marks the date in 1945 when the emperor visited Major Ishii's facility.
'The emperor knew what was happening here,' says Watanabe. 'And there are also photos of him visiting another facility near here where they researched poisonous gas as a weapon.'
The Allies decided not to prosecute the emperor at the end of the war as they decided he was more useful as a figurehead of the nation and as a bulwark against spreading communism in the Far East. Major Ishii escaped punishment for the atrocities he had committed, allegedly by divulging the know-how he had built up about banned weapons.
There are, unsurprisingly, many in Japan who do not want what went on at the Army medical college to come to light, but Watanabe is not one of them. 'Even today, we must take responsibility for what this country did,' he said.
In 1989, workers unearthed 62 skulls and other human bones
Some of the skulls had bullet holes in them. The bones were estimated to have come from this number of Asians: 100