An 80-year-old bomb dating from Spain’s civil war was detonated off the coast Barcelona on Monday, 26 August, after being discovered on a beach at the weekend.
It contained 70 kilogrammes of trinitrotoluene (TNT) and was so dangerous it had to be put in special diving bags and detonated 45 metres below sea level.
Officials at Barcelona City Hall said it had almost certainly been dropped by an Italian air force plane in he 1930s.
Benito Mussolini’s fascist regime supported General Francisco Franco's nationalists against the republican side, which finally surrendered Barcelona in January 1939.
TNT is just one of a number of explosive materials which have gone in and out of fashion over the centuries.
Gunpowder
In the ninth century Chinese monks seeking an elixir for long life, accidentally invented gunpowder by mixing saltpeter, sulphur and charcoal.
Gunpowder was, as the name suggests, mainly used in the barrels of guns in the Middle Ages but it was at times used to create early bombs, a science which was again pioneered by the Chinese and the Mongols.
In 1605 the most famous terrorist plot involving gunpowder came to nothing.
A group of English Catholics planned to blow up the Protestant King James I of England, along with the House of Commons and the House of Lords during the opening of Parliament.
They rented a basement and filled it with 36 barrels of gunpowder but before Guy Fawkes could light the fuse, he was discovered and the plot exposed.
Fawkes and the other conspirators were tortured and executed.
In the 19th century smokeless powder gradually replaced gunpowder as a propellant for guns and by then more efficient and deadly explosives had been invented.
Dynamite
In the 1860s Alfred Nobel, a Swedish engineer who built many of the bridges and buildings in Stockholm, invented dynamite while trying to find a material which could blow up rock to assist construction.
Dynamite is a mixture of nitroglycerin - a volatile chemical which had been invented by Italian chemist Ascanio Sobrero - and silica.
Nobel - whose name is ironically associated more now with the Peace Prize he later established - also patented a blasting cap.
Nobel moulded the dynamite paste into cylinders which could then be pushed into holes in the rock, lit with a fuse and detonated. Dynamite soon became popular in quarrying, road and rail construction.
Sticks of dynamite, usually red in colour, became the default image for explosives and remained popular with terrorists right up until the early 1980s.
In his book “Days of Rage”, author Bryan Burrough recalls how easy it was for members of the Weathermen, a US radical group, to purchase dynamite in the 1970s as they planned to bomb military and other establishment targets on the East Coast.
Burrough wrote: “The dynamite proved easy to secure. On Monday March 2 (1970)…Ron Fliegelman did what Sam Melville hadn’t realised he could do: he walked into the offices of New England Explosives in Keene, New Hampshire, presented the stolen driving licence of a New York rabbi and laid out less than $60; he walked out with two fifty-pound cases of American Cyanamid dynamite, each case containing one hundred sticks.”
Four days later, as they were preparing to bomb a dance at Fort Dix US Army base in Philadelphia, the device - made of dynamite and packed with nails - prematurely exploded in the basement of a townhouse in Greenwich Village, New York.
The bombmakers, Terry Robbins and Diana Oughton, were killed outright and a third member of the Weather Underground also died.
But the Weathermen, the Black Liberation Army, the Symbionese Liberation Army and the Puerto Rican FALN continued to use dynamite as the main ingredient for their bombs throughout the 70s and into the 1980s.
Gelignite
But dynamite was not the only explosive Nobel invented.
In 1875 he also came up with the idea for gelignite, which is nitrocellulose or gun cotton dissolved in nitroglycerine and mixed with wood pulp and saltpetre.
Unlike dynamite, which can become dangerously unstable if it gets hot or starts “sweating”, gelignite is a stable material which is much safer to handle.
Cheaper to produce, it will only explode if a detonator is attached.
In the 1970s the Provisional IRA stole a batch of gelignite, known by the trade name Frangex, from a factory in County Meath.
Patrick Magee, who was convicted of bombing the Grand Hotel in Brighton in an attempt to assassinate Prime Minister Margaret Thatcher, was found in possession of 3.5kg of Frangex gelignite.
IRA informant Sean O’Callaghan claimed the 1984 Grand Hotel bomb contained around 10kg of gelignite.
The IRA later switched its preference to Semtex but as recently as 2000 a batch of gelignite sticks was discovered on a farm in County Kilkenny and was believed to be owned by a dissident republican unit.
Gelignite has largely passed into the history books but in 2015 a huge explosion in the town of Petlawad in India killed more than 100 people. It was blamed on gelignite which had been illegally stored by a Hindu extremist who was linked to the son of a BJP politician.
TNT
Trinitrotoluene, better known by its initials TNT, was used during the First World War to fill artillery shells and trench-busting bombs.
Women workers at munition factories in Britain used to complain about the colour their hands turned after handling the yellow, odourless powder.
TNT was rarely used by terrorists but it was deployed on one infamous occasion in September 1920.
A huge TNT bomb was loaded it onto a horse-drawn cart which was led through the Wall Street financial district in New York.
It detonated at lunchtime, killing 38 people and injuring hundreds of Wall Street brokers, clerks and receptionists.
The bombers were never caught but a shadowy group called the American Anarchist Fighters were blamed. They are thought to have been linked to an Italian anarchist cell who carried out bombings in the US in 1919.
Semtex
Semtex, a mixture of RDX and pentaerythritol tetranitrate, was invented in Czechoslovakia in the 1960s and quickly became popular with terrorist groups around the world.
The communist regime in Prague were not backwards in furnishing groups like the IRA and the Palestinian PFLP with Semtex.
Semtex was used to make the bomb which blew up Pan-Am Flight 103 over Lockerbie in Scotland in 1988.
The name comes from Semtin, a suburb of Pardubice in what is now the Czech Republic.
In 1990 the President of Czechoslovakia, Vaclav Havel, said the country’s former communist rulers had sold 1,000 tons of Semtex to Libya.
He said: “If you consider that it takes seven ounces to blow up an aircraft, this means world terrorism has enough Semtex to last for 150 years.”
In 1997 Bohumil Šole, a scientist who claimed to have helped Semtex, blew himself up with it at a spa.
Plastic Explosives
The British Army developed plastic explosives during the Second World War, based on RDX.
Different recipes were tried until in 1958 the Phillips Petroleum Company, in the United States, came up with Composition 4 or C-4.
The explosive material in C-4 is cyclotrimethylene-trinitramine but it is bonded with plastic binder material which gives it a consistency like modelling clay, which enables it to be moulded into whatever shape is required.
The United States is the biggest manufacturer of C-4 but other countries, include Iran and Israel produce it commercially.
In October 2000 al-Qaeda used C-4 to attack the warship USS Cole off the coast of the Yemen, killing 17 sailors.
C-4 was also used four years earlier to blow up the Khobar Towers, a US militarty housing complex in Saudi Arabia.
Ammonium Nitrate
In the 1990s, with their supplies of Semtex drying up, the Provisional IRA began experimenting with ammonium nitrate bombs.
They were not the only terrorists to realise the explosive power of ammonium nitrate, a chemical used in everything from fertilisers to hair products.
The Oklahoma City bombing in 1995 killed 168 people, including 19 children. Timothy McVeigh's bomb contained 2,200kg of ammonium nitrate and fuel oil.
Insurgent groups used it in Iraq and Islamist extremists used it in various attacks in the UK, including on 7 July 2005 when rucksacks packed with ammonium nitrate exploded on a bus and on London Underground trains, killing 52 innocent people.
In 2004 police in London swooped on a terrorist gang who planned to explode a huge ammonium nitrate bomb in London, possibly near a shopping mall or a nightclub.
The Islamists targeted by Operation Crevice had bought 600kg of ammonium nitrate fertiliser from an agricultural merchants in Sussex and kept it in a self-storage facility in Hanwell, west London.
When mixed with other ingredients including aluminium powder and sugar it would have made a deadly device with devastating consequences in a crowded area.
Four men - led by Omar Khyam, 26 - were jailed for life for their part in the bomb plot.