Thousands of World War II bombs still lurk underground – and they’re even more dangerous now than in the 1940s, making defusing a risky, delicate process. Jon Excell investigates.

In August, construction workers in east London unearthed a terrifying relic from World War II: an unexploded 500lb (227kg) German bomb with the potential to obliterate surrounding homes and buildings.

The scene was immediately cordoned off, 700 people were evacuated and experts from the British army’s Royal Logistics Corps (RLC) Explosive Ordnance Disposal (EOD) unit spent a nervous 24 hours making the device safe. It was the third such incident in London in almost as many months. A smaller bomb was found by workmen in Wembley in May, while a giant 550lb (250kg) bomb found in Bermondsey in south London in March prompted the evacuation of more than 1,000 people.

During the Blitz, the Germans dropped about 24,000 tonnes of explosives. But 10% of the bombs didn’t actually detonate

According to the London Fire Brigade, the Bethnal Green device was the ninth unexploded WWII bomb found in the capital this year. And it won’t be the last.

Even now, 70 years after the war’s end, tens of thousands of similar items of unexploded ordnance (UXO) may still be awaiting discovery beneath the peaceful gardens, roads and buildings of post-war Britain, say experts. And they’re often more dangerous now than they were during the war – which makes defusing them a difficult and potentially risky process.

The reason they’re there at all is that a frighteningly large proportion of bombs dropped on Britain in World War II simply failed to go off. “During the Blitz between September 1940 and May 1941, there were about 85 major raids on London, and during those raids the Germans dropped about 24,000 tonnes of high explosive,” says Matt Brosnan, a historian with the Imperial War Museum. “But 10% of bombs that were dropped didn’t actually detonate.”

Some bombs, Brosnan says, were simply duds – casualties of fast, furious and error-prone wartime manufacturing. Others may have suffered damage to the fuse typically used to detonate the bomb, or to the clockwork time-delay mechanisms that were sometimes used. All of them, if mishandled, could present a major hazard.

Bombs away

The Germans produced an array of bombs during the war, ranging from simple 110lb (50kg) devices to a giant 3,970lb (1,800kg) explosive nicknamed Satan. But the majority of those dropped on Britain – and the majority of those that are found today – were the so-called iron bombs, or unguided explosives delivered from aircraft, ranging from 110lb (50kg) to 550lb (250kg). Half of each bomb’s weight was the explosive charge; the other half was the bomb body’s metal, which would fragment upon explosion.

The German bombs were fitted with a variety of fuses, some designed to detonate immediately, some with a time delay and some which were booby trapped

The British Army is called out on almost daily basis to deal with many different types of World War II ordnance – including souvenir hand grenades brought back from Germany by returning troops, small arms ammunition and unused British and American ordnance.

But according to a spokesman for the RLC regiment’s EOD division, high explosive, air-delivered German bombs are the most dangerous World War II items it has to deal with. Not only are they in a sensitive state – having already been deployed, armed and damaged by the impact with the ground – but they are fitted with a variety of different fuses, some designed to detonate immediately, others which featured some form of time-delay and some which were booby trapped, specially designed to kill EOD operators.

They are also more unstable – and potentially lethal – today than they were 70 years ago, thanks to chemical degradation of the fuse. “Just the scrape of a workman’s shovel hitting the bomb body or the fuse packet could cause a chain reaction,” says Simon Cooke, a former British Army major and leading EOD specialist who heads up 6 Alpha Associates, a risk management consultancy specialising in explosives. “It would do it instantaneously. The whole thing would be gone in hundredths of a second and you would be dead.”

The bombs are more unstable – and potentially lethal – today than during World War II

The UK has been fortunate in its dealings with such a hazardous legacy. Although many EOD operators were killed during and immediately after the war, when army units dealt with around 45,000 unexploded devices, there have been no recorded casualties in the UK in recent years.

Sadly, this isn’t the case for the rest of Europe. Most recently, in 2014, one construction worker was killed and several others injured when a digger uncovered a British bomb in Euskirchen in north-west Germany.

This could be because British bombs haven’t aged quite as well and tend to be a little more volatile, suggests Cooke. But there’s no doubt that the UK’s approach to bomb disposal has also played a major role in keeping the public and the experts safe.

Disarming danger

So what happens when a bomb is discovered?

Some devices can simply be picked up, carried away and disposed of. Most, however, are in such a sensitive state that moving them is simply too dangerous and they have to be dealt with on site. “This weapon is more than 70 years old,” says Cooke. “It hasn’t gone off and you can’t be certain why. A jolt, a shock, a knock, a drop, a bang on the back of a truck, a suspension rattle going over a pothole could set one of these things off.”

This weapon is more than 70 years old. It hasn’t gone off and you can’t be certain why – Simon Cooke

What happens next depends largely on the size of the bomb. For smaller devices it’s often possible to build a protective structure around the bomb and detonate it where it lies. For larger items, the structure required would be so enormous that this approach isn’t an option.

Cooke likens the design of a bomb to the process of making a fire: you light a match, ignite some paper, set fire to some kindling and finally ignite the coal. In his analogy the match, the most sensitive component in the whole chain, is the fuse, and the coal the explosive. The first and most critical step in the so-called Render Safe Process (RSP) is therefore to deactivate the fuse. Fuse Immunisation, as it’s known, typically involves drilling into the fuse and introducing a solution that neutralises the chemicals.

As Dave Welch, a former Royal Navy bomb disposal officer, explains, this process varies depending on the type of fuse. Fortunately, says Welch, who now runs one of the UK’s largest private bomb disposal firms, Ramora UK, it’s relatively easy to identify WWII-era fuses from the codes that were punched into the fuse body during the manufacturing process. Once the fuse has been identified, the EOD officer can decide on the correct procedure.

For instance, if it’s a “number 17” (a time-delay fuse that could be set to detonate the bomb anytime between two and 80 hours after the bomb hit the ground), the procedure is to drill into the fuse and pump a saltwater solution through the fuse itself. This saltwater is left in the fuse for a certain amount of time and then expelled, leaving behind salt crystals that jam up the cogs, ensuring the timer will no longer work.

Interestingly, while there have been some technological developments – such as remote “stethoscope” equipment used to listen for the tell-tale ticking of a reactivated timer – the methods used to deal with WWII ordnance haven’t changed much since the war.

“The guys during the war were proper pioneers,” says Welch. “A lot of people died figuring out how these things worked. And because modern bombs are different, the WWII principles still apply to the WWII bombs, there’s no other way of dealing with them. There are slightly enhanced drills and pump systems for putting the salt water in, but it’s still the same process.”

A lot of people died figuring out how these things worked and how to defuse them – Dave Welch

Once the fuse has been neutralised, the bomb still has to be disposed of. Sometimes it is relocated to a remote site and detonated with modern explosives (as was the case with the Bermondsey device).

Alternatively, operators might drill a hole into the bomb casing itself and carefully steam out the explosive. “If you introduce wet steam into the bomb at a very carefully controlled high temperature, it will be just right to melt the high explosives out of it without raising the temperature to such an extent that it will cause the bomb to explode,” explains Cooke.

Unsurprisingly, the bombs found in the middle of densely populated areas are those that tend to generate the biggest headlines – and the development of the UK’s urban areas will continue to uncover such devices.

Perilous seas

But even the onshore problem is dwarfed by the vast legacy of unexploded ordnance that litters the UK’s seabed, says Cooke.

The waters around the UK resemble an explosive soup

During both World War I and II, millions of sea mines were laid as a defensive barrier. It’s estimated that between 30 and 70% were not recovered. Add to this the unexploded torpedoes launched by submarines, surplus bombs jettisoned by both British and German planes and the poorly regulated munitions dumping practices which began during World War I and only ended a few years ago and the waters around the UK begin to resemble an explosive soup.

For decades, none of this was much of a problem. But the huge growth in offshore renewables (particularly off the east coast of Britain) in recent years means that seabed ordnance is now becoming a major headache. “We’ve worked on 50 renewable energy or cable installation projects in last 10 years and every single one had a risk of encountering UXO,” says Cooke.

And while most subsea ordnance is at least so far offshore that it doesn’t present a risk to the public, this isn’t always the case. In the mouth of the Thames Estuary – in water so shallow that, at low tide, you can see its mast jutting out from the water – sits a sunken ship that is perhaps the most startling illustration of Britain’s offshore ordnance problem.

In the mouth of the Thames Estuary, the SS Richard Montgomery languishes with 1,400 tonnes of high explosive becoming more unstable by the day

Wrecked on a sandbank near Sheerness in Kent during a storm in 1944, the SS Richard Montgomery was a so-called US Liberty ship delivering much-needed ammunition to help in the war effort. But it never got to its destination. And years later it languishes with its deadly cargo of some 1,400 tonnes of high explosive becoming more unstable by the day.

“It’s full of artillery shells, and they’re all deteriorating and becoming more sensitive over time,” says Cooke. “They’ve hitherto been contained in the body of the vessel, but eventually that’s going to break down and the shells could spill from the ship’s body. If they do, and one goes off, all the others could detonate. It’s called sympathetic detonation. And if that occurs, it could be a catastrophe.”

Correction: An earlier version of this article misstated the amount of unexploded ordnance on the SS Richard Montgomery, as well as Dave Welch's former position; he was a former Royal Navy, not British army, bomb disposal officer.

Jon Excell is the editor of The Engineer.

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