When Sergei and Yulia Skripal were poisoned by nerve agent novichok in 2018, one of the most secretive sites in Britain suddenly found itself getting a lot of attention.
Porton Down, Britain's top secret military research centre, is just a few miles away from where the Skripals were found in a catatonic state on a park bench in Salisbury.
It was thanks to the expertise of its experts - who identified what nerve agent was used in the Russian-orchestrated attack - that the Skripals' lives were saved.
But, as a scientist who carried out three years of research in the base's archives revealed this week - there is a dark side to Porton Down too.
It was there in 1953 that airman Ronald Maddison died within hours of being exposed to the nerve agent sarin in a test.
Other servicemen who were among the 21,000 who were exposed to highly dangerous substances in tests between 1939 and 1989 have reported adverse health effects.
And figures obtained by MailOnline last year showed how more than 2,000 pigs, rabbits, monkeys, guinea pigs and mice have been killed in experiments at the base in the last three years.
Some tests have even involved pigs being blown up to test the effectiveness of soldiers' body armour.
Scientists wear gas masks and protective clothing as they carry out research at Porton Down, 1964
A warning sign outside Porton Down, near Salisbury in Wiltshire, in October 1968
Porton Down was set up in 1916 - two years before the end of theFirst World War - to research and create chemical weaponsin the wake of German gas attacks.
Initial research examined chlorine, phosgene and mustard gas, all of which had been used in war.
Research into offensive weapons was abandoned after the Second World Warand the focus instead turned to defensive capabilities.
Epidemiologist Dr Thomas Keegan, a lecturer at Lancaster University, was part of a team that researched Porton Down's archives for three years from 2002.
The archives had been entirely closed off, but Dr Keegan and colleagues were given access afterofficials decided to allow greater transparency amid the controversy over the test subjects who reported damage to their health.
Dr Keegan and his colleagues carried out his research in a building at Porton Down called Hut 42.
The expert recalled: 'Porton Down's record keeping was excellent. Early on I had worried that handwritten records would be hard to decipher and had asked a Porton Down librarian whether they would be legible.
'"Definitely", was the reply. "People had neat handwriting then. It's the records from the 1970s you'll have to watch. They're dreadfully scrappy," he said.
In 1953 RAF airman Ronald Maddison died within hours of being exposed to the nerve agent sarin in a test at Porton Down
A soldier is seen laughing while on the hallucinogenic drug LSD, which was given to him as part of a test. He was filmed as part of a test
Sergeant Gordon Beard of the RAF wears protective clothing and a residual vapour detector while at work at Porton Down, June 1969
Experiments were recorded in meticulous detail. Above: Diagrams depicting the testing of substances on a person's arm
'And so it was proved. The records of tests from an era before computers, carried out with substances such as mustard gas, were routinely neatly and clearly documented.'
Tests - and the subjects they were carried out on - were recorded in 140 books.
'Some were in the form of descriptive text, others used pictograms to record the site visually, for example where a drop of mustard gas was placed on the skin,' Dr Keegan wrote.
Also tested were vomiting agents that were intended to force victims to take off their gas masks, meaning they were vulnerable to another attack.
Troops who were given LSD were 'unable to put up much of a fight', Dr Keegan noted.
As for nerve agents, 3,000 people were exposed to them. As well as sarin, they included tabun and soman.
Exposure to tabun, which smells like bitter almonds, can cause death in minutes. As little as 1milimetre of it on the skin can kill.
Soman is also extremely toxic. Dr Keegan found that six people were tested with it in July 1951.
The cover of a report dated September 28, 1945, into the effects of a nerve agent on subjects' eyes
A page from a report detailing 'moderate tearing' on a subject's eyes after they were exposed to a nerve agent
According to lab notes, all experienced 'pain in the eyes' and 'blurred vision 30 minutes after exposure'.
Symptoms continued for 24 hours. After 48 hours, vision had improved but was still not back to normal.
It was not just chemical agents tested at Porton Down. Records uncovered by Dr Keegan showed that nearly 496 people were tested with 'lubricating oil' and 204 with ethanol.
RAF airman Ronald Maddison died at Porton on May 6, 1953.
A report in the Daily Mail the following month told how Maddison's death certificate had merely said that he 'died from asphysxia by blocking of the bronchial tubes'.
It was through a question in the House of Commons that the Government revealed Maddison had died in a 'war-gas trial'.
At a 2004 inquiry into the government's use of servicemen as test subjects at Portdon Down, the full details of Maddison's excruciating death were reported.
The soldier and five other servicemen were told they were helping to find a cure for the common cold.
A worker seen next to one of the furnaces used to destroy material at Porton Down, 1979
Chemists at work in a laboratory at Porton Down in July 1968
What scientists dripped onto a cloth wrapped around Maddison's left forearm was the nerve agent Sarin.
After half an hour, Maddison reported feeling 'perfectly well'. But, ten minutes later, he said he felt 'pretty queer'.
He then lost his hearing and started struggling to breathe. He was dead soon afterwards.
In 2006, Maddison's family received £100,000 in compensation from the Ministry of Defence after it was ruled that he had been unlawfully killed.
Other servicemen were exposed to mustard gas, CS gas (commonly known as tear gas), the nerve agent VX and hallucinogens such as the illegal drug LSD.
North Korean agents used VX in 2017 to assassinate the half-brother of the country's dictator, Kim Jong-un, in Kuala Lumpur.
In 2023, a study carried out by experts at King's College London and Lancaster University found that ex-servicemen who took part in chemical warfare research faced a six per cent higher mortality rate than those who did not.
And personnel who were part of trials between 1960 and 1964 were at a 34 per cent greater risk of early death.
A truck fitted with a tank designed to contaminate terrain with mustard gas, Porton Down 1941
Experts at work on the lassa fever virus at Porton Down in 1979
A range of conditions, including cancers, caused their deaths.
An MoD spokesman said at the time: 'We are grateful to all those individuals who took part in these studies and contributed to keeping our country safe.
'We will continue to work alongside King's Centre for Military Health Research and welcome the findings in this report.'
Overall, Dr Keegan revealed, more than 400 different substances were tested at Porton.
In the 1960s, Porton Down scientists even unleashed a bacteria on unsuspecting London Underground passengers.
Ministers reluctantly gave the go-ahead after being assured that there was not a great threat to health.
The bacterium, called bacillus globigii, were carried onto a Tube train on the Northern Line in a small tin.
At a midway point between Colliers Wood and Tooting Broadway, the train window was opened and the tin was dropped onto the track.
Porton Down is officially called the Defence Science and Technology Laboratory
The aim was to discover how far the bugs could travel to estimate the number of people who might be affected by the small container of bacteria.
Scientists tested air samples over the next two weeks and were shocked to discover that the bugs had travelled ten miles north. Trains also became 'heavily contaminated', a report said.
Bacillus globigii was not considered harmful at the time, but research over the last decade has revealed it is capable of causing food poisoning, fevers and septicemia.
A Daily Mail investigation in 2010 revealed that in the five years previously, 119 live pigs had been blown up with explosives.
Before being blown up, the animals had monitoring tubes inserted into their blood vessels and bladders.
And a major blood vessel in the abdomen of some pigs had a wire put into it so it could be lacerated during the blast to allow for a controlled bleed to simulate blood loss in a human blast victim.
The animals were aneasthetised but were left untreated for five to ten minutes after the charge was detonated to simulate the delay between injury and medical help on the battlefield.
Pigs also had sulphur mustard applied to their skin.
The tests were carried out to examine how barrier creams protect against the agent, which causes severe skin legions.
In another experiment, ten marmoset monkeys were exposed to inhalational tularemia, a disease caused by a bacterium that is used as a chemical weapon.
The experiment was set up to establish what is known to experts as the LD50 - the dose that causes 50 per cent of the animals to die.
Last December, a Freedom of Information request sent by MailOnline revealed how many animals have died in experiments in the last three years.
In all, figures showed 64 guinea pigs, 97 marmosets, 83 pigs, three rabbits and 1,797 mice were killed in experiments at Porton Down between 2021 and 2023.
The MoD said the tests were part of 'life-saving' research to help better protect British troops, and 'cannot be conducted without the use of laboratory animals'.
Porton Down is actually made up two government facilities. The military research centre is officially called the Defence Science and Technology Laboratory.
The second site is used by the UK Health Security Agency, which was set up in 2021 to replace Public Health England.
The campus has also housed the Porton Science Park since 2018.
Today, scientists at Porton Down undoubtedly do much to keep Britons safe and to enhance the welfare of troops on the battlefield.
And scientists at the site have also researched natural threats such as the deadly Ebola virus.
But the post-war legacy of tests on 'guinea pig' volunteers will not be forgotten by many.
An MoD spokesperson said: 'The death of Ronald Maddison in 1953 was a tragedy. We have acknowledged the verdict of unlawful killing from the 2004 inquest and compensation was provided to Mr Maddison's family.
'We understand that some veterans may have concerns about their participation in historical trials and can get in touch through our dedicated helpline on 0800 783 2521.'
