The massive explosion that devastated much of Beirut, which was felt as far away as Cyprus, struck with the force of an earthquake.
It was the most powerful blast ever seen in the Lebanese capital, which was on the front line of the country's 1975-1990 civil war and has endured conflicts with neighbouring Israel, as well as bombings and terror attacks.
Lebanon's government has said that the explosion was caused by the detonation of ammonium nitrate stored at the city's port. Experts believe this happened after a fire engulfed fireworks, which were also stored in the area.
The disaster has raised questions over why such material was kept for so long, in such an unsafe manner, so close to heavily populated areas. The substance has been the cause of numerous industrial explosions in the past.
What is ammonium nitrate?
Ammonium nitrate is an odourless crystalline substance commonly used as a fertiliser. In the agricultural sector, farmers buy it in bulk as its application allows nitrogen — which is key to plant growth — to be released into the soil.
It is not easily combustible: typically the compound does not detonate on its own but it can explode if it comes into contact with another ignition source, or if it comes under intense heat. Known as an oxidiser, it intensifies combustion and can enable other material to ignite more readily.
Trevor Lawrence, an expert on explosives and ammunition, told Euronews that "ammonium nitrate is a fairly stable substance as it is, but when it becomes contaminated, when it's not stored in dry and clean conditions, it becomes more volatile."
"Critically, when it's subjected to heat, and it's confined in large amounts it starts to decompose and it decomposes generating its own heat. So you get a rolling reaction, it gets hotter and hotter, decomposes more and more. And if it's combined, that can transition into a detonation as we saw yesterday," Lawrence said.
There are often strict rules as to where and how ammonium nitrate can be stored: for instance, away from fuel and other sources of heat, and away from residential areas.
"It has to be kept dry, it has to be kept cool and critically has to be kept in an area where it's not going to be subjected to any heat or flame," Lawrence told Euronews.
Many countries in the European Union require calcium carbonate to be added to ammonium nitrate to create calcium ammonium nitrate, which is safer.
What other large explosions has the substance caused?
Ammonium nitrate was implicated in an explosion at a chemical plant in the French city of Toulouse in 2001 that killed 31 people and was ruled accidental. The material was also used in a blast at a fertiliser plant in the US state of Texas in 2013, killing 15, that the authorities judged to be a deliberate attack.
Also in Texas, 581 people were killed in an industrial disaster in Texas City in 1947. A fire took hold as a consignment of ammonium nitrate fertiliser was being unloaded at the city's port. The resulting explosion was reportedly heard over 200 kilometres away and set off a chain reaction that destroyed ships, small planes flying overhead, and caused a tidal wave.
Ammonium nitrate was a component in the bomb used in the Oklahoma City bombing in 1995. Two far-right extremists, former US soldiers, blew up a government building, killing 168 people. Afterwards, regulations governing the use of the substance were tightened significantly.
The following year, it was used in the largest bomb ever detonated by the IRA in Britain. The attack in June 1996 caused huge damage to Manchester city centre. Prior warnings meant no-one was killed but some 200 were injured.
Lawrence told Euronews that it often comes down to poor storage.
"It's not properly monitored, it's not kept under proper conditions, then eventually there's a there's a really strong risk that this sort of event is going to happen," he said.
Why was ammonium nitrate stored in a populated area of Beirut?
Lebanon's Prime Minister Hassan Diab said 2,750 metric tons of ammonium nitrate had been stored in a warehouse at the port since it was confiscated from a cargo ship in 2014.
A lawyers' report from 2015 and published by shiparrested.com refers to one ship, the MV Rhosus, which was initially seized in Beirut in 2013 after encountering technical problems en route to Mozambique from Georgia.
The ship was carrying "2,750 tons of Ammonium Nitrate in bulk," it says, referring to the cargo's "dangerous" nature.
"Owing to the risks associated with retaining the Ammonium Nitrate on board the vessel, the port authorities discharged the cargo onto the port’s warehouses. The vessel and cargo remain to date in port awaiting auctioning and/or proper disposal," the report concludes.
Before Tuesday's blast, Lebanon was already on the brink of collapse. Amid near economic meltdown and soaring unemployment that have brought mass protests in recent months, many blame government incompetence and corruption.
The prime minister has vowed that "those responsible will pay", following Tuesday's disaster.
Watch The Cube's investigation into what happened to the MV Rhosus and its cargo below: