Sarin, a deadly nerve gas which a UN rights investigator says Syrian rebels may have used in their fight against President Bashar al-Assad's regime, was developed by Nazi scientists in the 1930s.
Originally concocted as a pesticide, sarin was notoriously used by Saddam Hussein's regime to gas thousands of Kurds in the northern Iraqi town of Halabja in 1988.
A cult also used the odourless, paralysing agent in two attacks in Japan in the 1990s.
Now, there is evidence Syria's rebels may also be employing it in their bid to overthrow Assad.
"According to the testimonies we have gathered, the rebels have used chemical weapons, making use of sarin gas," UN human rights investigator Carla del Ponte said on Sunday.
Inhaled or absorbed through the skin, the gas kills by crippling the respiratory centre of the central nervous system and paralyses the muscles around the lungs.
The combination results in death by suffocation.
"Sarin is 26 times more deadly than cyanide gas. Just a pinprick-sized droplet will kill a human," according to the World Health Organisation.
Exposure symptoms may include nausea and violent headaches, blurred vision, drooling, muscle convulsions, respiratory arrest and loss of consciousness, according to the US Centers for Disease Control and Prevention.
Nerve agents are generally quick-acting and require but simple chemical techniques and inexpensive, readily available ingredients to manufacture.
Inhalation of a high dose -- say 200 milligrammes of sarin -- may cause death "within a couple of minutes," with no time even for symptoms to develop, according to the Organisation for the Prohibition of Chemical Weapons.
Exposure through the skin takes longer to kill and the first symptoms may not occur for half and hour, followed by a quick progression.
Even when it does not kill, sarin's effects can be permanent -- damaging a victim's lungs, eyes and central nervous system.
Heavier than air, the gas can linger in an area for up to six hours, depending on weather conditions.
The most notorious attack occurred in March 1988 in Halabja when as many as 5,000 Kurds were killed and 65,000 injured when the Iraqi military used a combination of chemical agents that included sarin, mustard gas and possibly VX, a nerve agent 10 times more powerful than sarin.
It is thought to have been the worst-ever gas attack targeting civilians.
Sarin killed 13 people and injured 6,000 others when the Aum Supreme Truth cult released it in the Tokyo subway in March 1995. The cult also used the nerve agent in an attack the year before in the Japanese city of Matsumoto, killing seven.
The Syrian regime is believed to hold hundreds of tonnes of various chemical agents, according to Leonard Spector of the Monterey Institute of International Studies.
In addition to blister agents known as vesicants such as mustard gas (yperite), Damascus is thought to possess sarin and possibly VX.
The Syrian regime is also said to possess delivery systems for its chemical weapons, such as Scud missiles, artillery shells and aerial bombs.
However, Damascus has refused to allow UN experts access to investigate the chemical weapons allegations despite appeals by UN chief Ban Ki-moon.