It's been described as among the deadliest chemical weapons ever made.
But Novichok, the substance confirmed by UK Prime Minister Theresa May to have been used in the attempted murder of Russian ex-spy Sergei Skripal and his daughter Yulia, is not merely lethal, it's also highly unusual.
It's so unusual in fact, that very few scientists outside of Russia have any real experience in dealing with it.
The substance, which means "newcomer" in Russian, was first developed in secret by the Soviet Union during the Cold War in the 1980s, as a means of countering US chemical weapons defenses.
Its existence remained secret until the mid-nineties, when information regarding its production was revealed as part of a deliberate leak by disgruntled Soviet scientist and whistle-blower Vil Mirzayanov. Even today, no country outside of Russia is known to have developed the substance.
It's that information that helped the UK Government conclude it was "highly likely" that either Russia tried to kill Skripal directly, or it had lost control of the nerve agent.
'Russian hands all over it'
Speaking to CNN, chemical weapons expert Hamish de Bretton-Gordon described Novichok as "a very sophisticated chemical weapon" that only a very select number of states would be capable of handling.
"It is difficult to imagine a scenario that doesn't have Russian hands all over it," said de Bretton-Gordon, a former commanding officer of the UK military's Chemical, Biological, Radiological and Nuclear (CBRN) Regiment.
"So, the chance that perhaps some of these Novichoks have been stolen by criminals or terrorists from Russia is a possibility, and we wait to see an explanation from the Russian Ambassador to London tomorrow, but I think highly unlikely."
What is Novichok?
According to Professor Gary Stephens, pharmacology expert at the University of Reading, Novichok is a more "dangerous and sophisticated" nerve agent than sarin which has been used in chemical weapons attacks in Syria, or VX, which was used to assassinate Kim Jong Nam, the brother of the North Korean leader Kim Jong Un, at an airport in Malaysia last year.
It is also harder to identify. "One of the main reasons these agents are developed is because their component parts are not on the banned list," said Stephens, referring to the Chemical Weapons Convention, an international arms control treaty signed in 1993 that prohibits the use, development, production, stockpiling and transfer of chemical weapons.
"It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier," said Stephens.
A colorless, odorless and tasteless liquid, Novichok would be easy to transport, suggested de Bretton-Gordon, who described it as likely going undetected "through an airport or a seaport or even through the mail."
How it works
Like other nerve agents, Novichok works by causing a slowing of the heart and restriction of the airways, leading to a slow and often painful death by asphyxiation.
Andrea Sella, professor of chemistry at University College London, described Novichok as causing a "systemic collapse of many bodily functions."
"In essence what they do is to block the mechanism that allows a nerve to reset itself after a signal has been transmitted," said Sella, adding that the symptoms were largely consistent, "especially the labored breathing and the muscular rigidity."
There is no treatment, only supportive care, including oxygen, anti-seizure medication, atropine, used for some poisoning patients, and pralidoxime chloride, given to inhibit poisoning including by nerve agents. Even just small doses can cause confusion and drowsiness.
"Treatment involves supporting breathing and delivering pharmaceuticals that on the one hand moderate the behavior of the nervous system and that can reverse the action of the agent," said Sella.
"It is likely that there will be long-term neurological problems for a person who has been exposed to these agents."
Skripal, 66, and his daughter Yulia, 33, remain hospitalized in critical condition. A police officer who came into contact with the nerve agent, Detective Sgt. Nick Bailey, remains hospitalized in serious condition but has been speaking to visitors.
How and where it was ingested
The next question facing the UK government is how precisely the attack was administered -- and how the Skripals ingested the agent.
According to the medical handbook, Responding to Terrorism, published in 2010, Novichok nerve agents "may be dispersed as an ultra-fine powder as opposed to a gas or a vapor," with the main route of exposure thought to be by inhalation, although "absorption may also occur via skin or mucous membrane exposure."
According to the book's authors, professors Ian Greaves and Paul Hunt, the Novochok class of agent was reportedly engineered to be undetectable by standard detection equipment, meaning further investigation may prove to be difficult.
On Friday, an additional 180 military personnel were deployed to the scene to help police investigate several sites amid concerns over potential contamination.
This was followed yesterday by the removal of the restaurant table where Sergei and Yulia Skripal ate on the day of attack for examination, a source confirmed to CNN. Restaurant employees were also advised to wash their uniforms, but not to burn them.
However, in spite of the extreme toxicity of these compounds, there would be very little risk to the general population, suggested professor Salla.
"There is no way of spreading the material around and it would decompose relatively swiftly in damp conditions," said Salla, who described the attempted murder as a "highly targeted attack."