It's 1917 and you're a British soldier fighting on the Western Front.
The bloody Battle of Passchendaele is raging near the Belgian city of Ypres in West Flanders.
The First World War is marching on with seemingly no end in sight. You're cold, tired, hungry and scared.
To make things even worse a shell has just landed in your bunker... There's something different about this one though.
There's no explosion flash or shrapnel flying. It almost feels like everything is going to be ok. But it won't be...
You start to smell a garlic-y odour around you. You’ve just been hit by one of the first mustard gas attacks in history.
Hours later you experience severe swelling of your eyelids, your skin starts reddening and horrible blisters start forming.
Your throat and chest become intensely sore and you start vomiting a noxious yellow fluid.
You feel queasy and will later suffer from terrible diarrhoea.
Things are looking bad but you're probably going to make it out of this one.
Listen to the third episode of 'Weapons That Changed The World' in full below...
It's estimated that only 1-5% of those unlucky enough to experience an attack actually died as a direct result of mustard gas.
Tens of thousands of tonnes of far deadlier gases, like chlorine and phosgene, had been used widely beforehand during the war.
Those unlucky enough to die from mustard gas developed more serious throat and chest infections and found themselves unable to open their eyes.
The power in the weapon didn't come in its lethality but in the fear it caused amongst the men.
Mustard gas could strike at any time. Your gas mask – which you'd be lucky to have – couldn't protect you from the gas engulfing your skin.
Many didn't even realise they'd been hit by mustard gas until symptoms started appearing hours later.
Although the gas was unlikely to directly kill you it would still leave you in an incredibly vulnerable position.
Bis(2-chloroethyl) sulphide is synthesised by treating sulphur dichloride with ethylene.
Although not used as a weapon until much later, mustard gas was likely synthesised as early as 1822, but it wasn't until 1860 when its dangerous properties were documented.
The Germans first used it as a weapon in 1917 but the Allies were quick to catch on.
Just a few months later the British were dropping it onto German trenches at Cambrai after they captured a stockpile of German mustard shells.
The breakout through the Hindenburg Line in 1918 was aided by a massive home-produced Allied mustard gas attack.
The Americans also started developing the poison that year.
Mustard gas is only slightly soluble in water, meaning washing it off is very difficult.
Worse yet, the gas will react with water to form a breakdown product called hemi-mustard, which is equally toxic and also releases hydrochloric acid.
According to the Imperial War museum, 95 men died as direct result of exposure to mustard gas in the first attack.
Many more were incapacitated, some with severe burns or respiratory problems.
Casualties had all their clothing removed and disinfected.
They were then washed and blistered skin treated with bicarbonate of soda. Their eyes were washed with a mixture of Zinc, boric acid, bicarbonate of soda and - cocaine.
To avoid contamination, medical staff wore protective clothing and respirators.
Fear has always played a role in the use of chemical weapons - which date back thousands of years - during warfare.
Ancient Hindu law permits the poisoning of food and water for use in battle as early back as 400BC.
In Greek mythology Hercules poisons his arrows with the venom of the Hydra monster, a tradition used throughout history by civilisations like the Ancient Romans and the Greeks, all the way to New World Native Americans.
The Viet Cong would poison bamboo sticks by wiping them in excrement so that when an American fell into a 'punji' trap they'd suffer nasty infections.
The psychological impact of chemical weapons on soldiers of any era has always been profound.
Death is even more terrifying when you often don’t know what's attacking you.
The unspeakable horrors caused by mustard gas led to widespread outrage at its use. The Daily Mail published an editorial attacking:
"The cold-blooded deployment of every device of modern science."
The terror faced during World War One and the remorse felt afterwards led to the ban of mustard gas in the 1925 Geneva Protocol.
The first chapter, on banning chemical weapons, read:
"The use in war of asphyxiating, poisonous, or other gases, and of all analogous liquids, materials, or devices, has been justly condemned by the general opinion of the civilized world…"
Despite the ban, the civilised world carried on suffering due to mustard gas.
Atrocities range from the Japanese using it against the Chinese in World War Two, to coalition forces encountering it in roadside bombs during the Iraq War, to the so-called Islamic state using it against the Syrian Army and Kurdish forces.
But mustard gas doesn't just impact people during wartime.
In 2017 scores of gas canisters had to be removed by Royal Navy and Army bomb disposal teams from Lincolnshire after two people were treated in hospital having discovered an abandoned stockpile.
Defence analysts have claimed that no one truly knows exactly how many mustard gas canisters could be left out in the countryside.
The canisters were sent to Porton Down. The secretive science complex was founded by the British as a direct reaction to the Germans' use of chemical weapons.
Opened in 1916 as the War Department Experimental Station tests began on human "guinea pigs' to determine the effects of chemical and biological weapons.
The laboratory also conducted research and development on chlorine, phosgene and mustard gas.
The research facility sent hundreds of Indian soldiers into gas chambers and exposed them to mustard gas, according to documents leaked to the Guardian newspaper.
Many British soldiers have alleged that they were duped into taking part in the tests, which have damaged their health in the years after the trials.
Yet much of the research conducted at Porton Down likely helped combat the deadlier effects of chemical weapons used against British soldiers.
On the flip side, the research also helped the British create even deadlier weapons of their own.
The UK's chemical and biological weapons programme was closed down in the 1950s.
Despite the many deaths it has caused the gas has also, surprisingly, saved lives.
Medical researchers aware of mustard gas' cell-destroying properties created the first cancer-fighting chemotherapy treatments using it.
Some argue that the use of a gas that would hospitalise people was preferable to using one that would certainly kill them.
Yet there seems an innate feeling amongst many that chemical weapons are somehow different – that they exist in a different moral category to conventional weapons.
But is it really worse to be killed by a chemical weapon than a conventional one?
There are likely as many different opinions on this as there have been chemical weapons, but one thing is clear.
The legacy of chemical warfare that mustard gas helped develop, still exists in a number of forms.
As recently as last month, Russian national and former spy Sergei Skripal was poisoned in Salisbury using a nerve agent.
We now live in a world where despotic leaders can use sarin gas indiscriminately on their own people, as in Syria, while protesters from China to Palestine to the United States are controlled using tear gas - a weapon banned in war but not for the police.