The unexploded Second World War bombs that are slowly growing more dangerous

Ordnance left behind by conflicts, such as this unexploded bomb found in Wembley, north London in 2015, can become deadlier with age – Sergeant Rupert Frere/Ministry of Defence

It would be nice to think that bombs become less dangerous with age. The worst time to encounter one is when it is falling towards your head, primed to blow. After that, their threat is diminished. The rusting old thing found by an excitable child in a quarry is something to be heeded, but less intimidating than when it was dropped by the Luftwaffe in 1940.

Or so you might have hoped. Sadly for shell-thumpers and grenade scrumpers, an increasing body of evidence points to the idea that such items might in fact become even deadlier with the passage of time. A recent Norwegian study of unexploded shells found that Amatol, a type of explosive common in Second World War ordinance, not only doesn’t become less explosive with age, but can in fact become even more so.

“Based on our findings, we can say that it’s relatively safe to handle, but you can’t handle it like TNT,” says Geir Petter Novik, a scientist at the Norwegian Defence Research Establishment. “It can certainly go off if it’s dropped, as opposed to TNT.” His study involved dropping weights from different heights on different types of explosive. Previous work had tested TNT and PETN, two of the other most common types of explosive, and found there was no drop-off in explosive properties, meaning they remained stable.

Construction workers uncover dud bombs that fell on London during the Blitz to this day – H F Davis/Getty

Amatol, a mixture of TNT and ammonium nitrate, was invented by the British Royal Arsenal in 1915, during the First World War, when supplies of regular TNT began to dry up. It was extensively used during the Second World War. The midget submarines that helped to sink the German battleship Tirpitz used Amatol-based mines; Amatol was also in the German V-1 and V-2 rockets. The findings on Amatol have ramifications for the global business of bomb disposal, estimated to be worth more than £4 billion a year.

“The thing to bear in mind is that bombs have never made themselves safe, and never have,” says David Welch, the senior explosives officer at Ramora Global, a leading disposal company. “It’s not a thing that bombs do. They’re an energetic article that is meant to function in a variety of ways, with a chemical reaction to cause heat, fire, damage and blast. They all deliver it in different ways and they all have a different mixture. But there’s no mix that you would look at and think, ‘let’s leave that for 100 years and it will be safe.’

“Amatol is not unique in maintaining its power over time, it’s just more available to becoming a gas [i.e. explode],” Welch adds. “There have been enough studies now to show that certain explosive mixtures are becoming more sensitive to interactions.” It is bad news if you’re the sort of person who might be tempted to bang on a shell casing with a trowel.

Controlled explosions are the stock-in-trade for Welch and his team

Welch served in the Royal Navy before moving into the commercial sector. “Very quickly, I realised I like things that go bang,” he says. “I started as a weapons engineer, then I became a diver, then I decided I wanted to shift to the disposal sites, as a mine clearance diving officer.” From the sea he moved on to land. He left the military in 2005 to do the same work as a civilian. He is still employed by the Armed Forces and works pro bono for emergency services, as well as private companies around the world. He estimates that one in 10 explosives fail to detonate correctly, which means there are millions of unexploded items around the world. Many are in obvious hotspots – Iraq, Afghanistan, Egypt – but also less obvious places, such as Belgium. It was only recently that the Falklands was made safe after the 1982 war.

Recent high-profile cases in the UK include detonations at Great Yarmouth and Plymouth, where German SC250 and SC500 bombs were disposed of. At Great Yarmouth, in February last year, the bomb detonated during the attempt to destroy it, causing a huge explosion that reportedly shook buildings 15 miles away.

Not all their work is clearance, however. When the MSC Napoli container ship foundered and was run aground on the Devon coast in 2007, it was Welch and his team who blew it up for salvage. “If it’s explosive related, we’ll do it,” he says.

Thanks to Novik’s findings, his team will change how it clears sites – meaning they will have to transport smaller quantities at a time, now that Amatol’s “impact sensitivity” is increasing (a result, he believes, of sensitive crystals or salts forming when the ammonium nitrate reacts with the bomb’s metal casing). In particular, Welch hopes the growing evidence might encourage caution on building sites, where unexploded munitions are often found and where construction workers can sometimes not take as much care as might be ideal. He says his business deals with unexploded ordnance in the UK on a “weekly basis”, with everything from small investigations to “pyrotechnics”.

Unsurprisingly, he says that the highest density of unexploded ordinance is in the south-east of England, which suffered the worst attacks in the Second World War. But unexploded German bombs are only part of it. He and his team might also be called on to deal with boxes of Home Guard hand grenades, or an old demolition ground that was not correctly cleared.

The aggregate industry can be particularly dangerous. To get building materials, ships dredge the bottom of the Solent or English channel, where the seabed is studded with unexploded munitions from the two world wars. The sand and aggregate piled up on the shore can be studded with potentially deadly surprises. The method of disposal depends on the situation: when the army encounters something in a large empty field it can simply blow it up. A civilian construction firm that finds a bomb next to a gas main is a very different proposition: the device may need to be moved before being detonated, with extra consideration given to noise and disturbance. Other techniques can help to defuse old explosive devices: drilling into the fuse or filling a clockwork fuse with salt water to clog it up. “There’s always a risk it might go bang,” he says. “But you’re making it as safe as you can.”

‘There’s always a risk it might go bang’ – Richard Watt/Shutterstock

Amatol has now been phased out of use, but it still lurks in old munitions, some of which are being used on the front lines in Ukraine and Russia.

“The type of warfare is different as well,” Welch says. “The old-school way of thinking was a mine every few metres. They are doing multiple-stacked mines, booby traps, all kinds of other issues. It will take just such a long time to find it all and allow people back in.”

Nobody knows when the war will end, but it could be another hundred years after that before the explosive-laden battlefields are fit for human occupation again. In the meantime, Welch and his colleagues will continue to do the perilous work of making ordnance safe wherever it is found: be it a Billericay building site or a village in Helmand.

The new research on Amatol is only more evidence of what he never forgets. “It’s nice to remind people that these are dangerous,” he says. Bomb disposal combines many different disciplines: chemistry, history, metallurgy, psychology, geography. But above all, it is a matter of life and death.

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