Video report by ITV News Security Editor Rohit Kachroo
The cause of the huge explosion in Beirut on Tuesday appears to have been the highly reactive chemical ammonium nitrate.
Lebanon President Michel Aoun said 2,750 tonnes of ammonium nitrate being stored unsafely in a warehouse at the dock caused the explosion.
Lebanese Red Cross officials said at least 100 people had been killed and more than 4,000 injured, and they warned the toll could rise further.
What is ammonium nitrate?
Commonly known as saltpetre, ammonium nitrate is a naturally occurring, highly soluble white crystalline solid.
The largest deposits of the odourless chemical are found in the Atacama Desert in Chile, from where it was mined.
On its own, ammonium nitrate is not usually regarded as particularly volatile or dangerous, however under certain conditions it can be deadly.
Most countries have regulations controlling its storage to make sure it is safe.
Explosives expert at Cranfield University, Trevor Lawrence, told ITV News: "Ammonium nitrate is a very common industrial chemical.
"The primary use is as fertiliser, most virtual fertilisers contain ammonium nitrate and it has a second use as a constituent of an explosive."
Mr Lawrence said: "It's not in itself considered an explosive but it is an energetic material," before later adding, "it has been used as the oxygen provider for an explosive as it produces oxides."
When asked about how it could cause such a large explosion and the regulations of ammonium nitrate, Mr Lawrence said: "Absolutely and it is strongly regulated throughout most of the world."
"But sadly this is not an uncommon occurrence, there have been a number of occurrences throughout the years of these large storage ammonium nitrate detonations, the culprit is always poor storage conditions," he added.
What can it be used for?
Ammonium nitrate is predominantly used in agriculture as a high-nitrogen fertiliser.
It has also been the cause of numerous industrial explosions over the decades, including at a Texas fertiliser plant in 2013 that killed 15 people, which was later ruled deliberate.
Under most conditions, the substance is said to be relatively stable and is inexpensive to manufacture, making the chemical a popular alternative to other, more expensive nitrogen sources.
How can ammonium nitrate explode?
The chemical is a key component of ANFO, (ammonium nitrate fuel oil), an industrial explosive used in mining, quarrying, and civil construction.
It also accounts for 80% of all the industrial explosives used in US.
So when combined with fuel oils, ammonium nitrate creates a potent explosive widely used by the construction industry but also by insurgent groups like the Taliban for improvised explosives.
How did ammonium nitrate arrive at Beirut port?
The vessel which is said to have had the ammonium nitrate on board was sailing from Batumi in Georgia to Biera in Mozambique in September 2013 when it had to enter Beirut port due technical problems.
After inspection by the Port State Control it was forbidden from sailing.
While in port, a number of creditors made claims against the ship through a Lebanese law firm called Baroudi and Associates.
The same law firm acted on behalf of the remaining crew who weren't initially allowed to leave the ship.Once the crew were granted permission to leave Beirut, the port authorities took charge of the cargo - 2,750 tonnes of ammonium nitrate - for fear of keeping it on the unsafe vessel.The ship was operated and owned by an Igor Grechushkin - a Russian national in Cyprus.
Has this type of ammonium nitrate explosion happened before?
There have been a few other instances of ammonium nitrate explosions.
Clifford Jones, visiting professor in chemical engineering at the University of Chester, said: “One of the best known was in Texas City in 1947, when there were more than 580 deaths.
“A much more recent one was at Waco, Texas, in 2013, when there were 15 deaths and 160 injuries.”
Other cases include in Oppau, Germany, in 1921, which resulted in more than 500 deaths, and Tianjin, China, in 2015, where a series of explosions involving ammonium nitrate and other chemicals killed 173 people.