Why bombs are still around decades after being dropped.
Hello again to all my readers. I started writing this week’s post as I flew home, full of glühwein and good cheer, from a weekend in Cologne. But this article isn’t about the German Christmas markets, delightful though these are. It’s about unexploded ordnance (naturally). I got thinking about it because my friend, who lives in Germany, has had to endure several bouts of evacuation over the years when unexploded WW2 bombs were found during construction. Germans take such things as a matter of course, but my friend is an expat and was definitely not used to this type of disruption where she came from. Thinking it was another example of German officiousness, she asked me (and I’m paraphrasing slightly):
“Why do they take such extreme precautions? I mean, it’s been in the ground for eighty years and hasn’t gone off yet. It’s hardly going to go off now… Is it?”
If you’ve ever wondered the same thing, then read on to find out why explosive ordnance disposal (EOD) is such complex work. In essence, the risk comes from uncertainty. Bombs are surprisingly intricate machines, and, with so many lying dormant for so long underground, there are many bad things that can happen. EOD teams have many ways of dealing with unexploded bombs1, but the most important part of any EOD operation is getting everyone out of harm’s way, and this is where the disruption comes in.
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Caution comes from uncertainty
If you’re not sure about something you should take extra precautions. This is especially true in EOD, for obvious reasons. Doing EOD with historic munitions always means dealing in uncertainty. Bombs, even WW2 era ones, are more than just big containers of explosives. They are sophisticated machines with many moving parts. Like all machines, there can be uncertainties in manufacturing and reliability which get compounded by decades spent underground.
Getting bombs to explode is surprisingly complicated
Bombs, like all munitions, are engineered to stay safe and inert (with minimal maintenance) for months or years in storage, and then detonate violently when dropped on a target:
The trick to achieving this is to cleverly manipulate the explosive train. Recall in our Expendables article that an explosive train is how you get from a small initial impulse to a large and violent detonation:
As you can see above, most of the complicated parts of the explosive train are in a part of a bomb called the fuze. The rest of the bomb is simply additional bang, with some fins for aerodynamic stabilisation and a lifting lug or two. The bomb shows is a German “Sprengbombe Dickwandig 50”, which means “thick-walled explosive bomb”. The “50” refers to its weight in kilograms2, and the series ran up to SD-1700, which, as you might guess, was a much bigger 1.7-tonne monster. Of course there are aerodynamic and weight and structural design challenges with making bigger bombs, but the fuzing—the complicated part—can be identical, so all you’re adding is marginal “bang”.
To keep bombs safe in storage (and in fires, plane crashes, bullet impacts etc.), the fuze uses mechanical and electrical components to keep the explosive train “out of line”, so that the accidental initiation of a sensitive primary explosive detonator does not set off the rest of the train and the whole bomb. An elaborate sequence of events needs to happen in the right order for the bomb to detonate. A typical example might be:
A bomb will not reliably detonate by simply dropping it from a great height. Note my use of the word “reliably”. It might, so don’t go dropping bombs out of planes on top of people just because I said it was okay (dropping anything heavy onto people from planes is not okay; bombs more so). A lot will depend on the type of explosive used3, but it’s certainly not a reliable way to get the bomb to go boom.
But as you know, we put fuzes on bombs for this very purpose. So why do they fail?
Mass production means many failures
To put it simply, when we make lots of something, then we will see every type of uncommon but possible failure. This is related to Murphy’s Law (“If there is a wrong way to do something, then someone will do it”) as well as the much less fun statistical Law of Large Numbers.
This is made more difficult when one design principle (such as safety, a.k.a. “will it explode unexpectedly?”) works in a contradictory way to another (e.g. reliability, a.k.a. “will it explode on the target?”). A feature which boosts the effect of one principle will detract from the other:
A very simple example of this would be a removable fuze, like we saw above for the German SD-50. You store the fuze and the bombs in separate storehouses4 so that if the fuzes explode accidentally due to having all that scary primary explosive inside, then the bombs will still be fine and you’ll be saved a much bigger and more catastrophic explosion. And, of course, you screw the fuzes into the bombs before the flight. But you can guarantee that someone, somewhere, will forget to put on the fuze, or will not screw it in far enough, or do any one of dozens of things which will prevent the bomb from exploding when it hits the target. So you’ve made it safer, but marginally less reliable5.
Leaving aside human error, there are all kinds of unlikely but possible mistakes that can arise during manufacturing. As with everything that’s manufactured, cost is a major factor, as shown above.
During World War Two, the Allies dropped over a million tonnes of bombs on Germany, and Germany dropped 40,000 tonnes on the UK during the Blitz. Taking a very rough average bomb size estimate of 100 kg, that works out at ten million and 400,000 bombs, respectively. That’s a lot of bombs. The estimated failure rate of these bombs was 10%, for all the reasons we discussed above. Most of these failures would have been found and dealt with at the time by EOD teams, but many were not. A half-tonne bomb falling from the sky can make a big hole, even if it doesn’t detonate. So it’s not a huge surprise that many of these failures were forgotten about in the chaos and confusion of blitz, destruction, and subsequent reconstruction. This is why these bombs keep surfacing, often literally, during construction works in German and British cities. Adam Higginbotham (of Midnight in Chernobyl fame) wrote an excellent article for the Smithsonian Magazine explaining the technical and human side of UXOs in Germany. It contains an excellent graphic showing how a mechanical-chemical fuze with an anti-tamper device works, and also how the same fuze failed to initiate the bomb when it penetrated the soil and came to rest nose-up:
Lest we think that this is a uniquely WW2 phenomenon, Belgian farmers still unearth 150 tonnes of WW1 UXOs from their fields every year. Modern weapons such as cluster munitions can have even higher failure rates, with an estimated 25% of the four million submunitions6 dropped on Lebanon during the 2006 war remaining as UXOs after the conflict.
Now that we’ve seen why bombs land without exploding, let’s consider the effect of the subsequent time spent buried underground.
Decades underground does strange things to a bomb
Here are two pictures of a German SC500 bomb. The first is from a recent EOD operation, and the second is a contemporary photo:
What are the differences? Perhaps it’s more apt to ask what the similarities are, since there are so few, at least on the face of it. But the EOD team dealing with it has to figure out its identity as a vital first step to dealing with it. They do this be measuring its length and diameter, overall shape (despite the mud and corrosion), and any markings (which are unlikely when the bomb is in that condition). They will also consult old records of air raids to see what types of bombs were used where and when. There’s also quite a lot of cooperation these days between German and UK authorities on the technical specifications of their own WW2 bombs and fuzes, since these keep coming up in the others’ cities.
Identifying the bomb is important because the EOD team needs to know:
- What is the main explosive? This is important because it has the potential to do the most damage to people and property.
- What other explosives are present as part of the explosive train? This is important because time and harsh environments can do funny things to explosives and make them unpredictable.
- How does the fuze work? This is important because it tells the EOD team what to do and what not to do. E.g. for an impact-initiated fuze, don’t kick it7.
Time and environmental factors affect these three things. In some cases, accidental detonation is less likely, e.g. if rust seizes up a clockwork fuze so that the firing pin can’t physically strike the primary explosive. In other cases, an accident is more likely, e.g. a safety pin corrodes and allows a detonator to come into line with a booster. The problem for EOD teams is that they don’t know what condition the fuze and the explosives are in. You saw the pictures above: these things will be caked in rust and dirt. X-rays might help a little, but only if the bomb is sitting in a favourable position to take an x-ray image (for instance, this would be very difficult or impossible in the case of the SC500 above). WW2-era bombs have been known to detonate if handled roughly, often with tragic results. For example, a digger struck a buried bomb in Germany in 2014, detonating it and killing the operator. Hitting a bomb with a digger would normally not cause it to initiate, but some combination of the state of the fuze and the environmental degradation meant that it was much more sensitive.
In summary, then, there are a few questions which EOD will most likely be unable to answer:
| Question | Can we answer it? |
|---|---|
| What is it? | Almost certainly |
| How is the fuze supposed to work? | Probably |
| Why didn’t it work? | Unlikely |
| What state is it in now? Is it safe to move? | Unlikely |
Once a bomb is (literally) unearthed, the (literal) alarm bells ring and a police and EOD job kicks off to answer these questions. That’s what we’ll talk about in the next section.
Getting a bomb out of a built up area is unsurprisingly complicated
When police discover a bomb, they will call a specialist EOD team. These might consist of military specialists, as in most of the UK, for example, or they could be civilian, as in Germany. The EOD team’s priorities will be protecting life, protecting property, and restoring normality, usually in that order8.
A long, small bang is preferable to a short, big bang…
A bomb, fundamentally, is a store of chemical energy. It’s designed to release that energy all at once on a target. When an EOD team finds an unexploded bomb, they want to release the energy somewhere else besides the target, which means taking the bomb somewhere remote and detonating it there. If they can’t do that, usually because the bomb is deemed unsafe to move, then they might try the next best thing and try to release the energy slowly, either by burning it out or by doing a controlled “low order” explosion, which is a non-technical term for anything less than a full detonation. Here’s an examples of what I mean:
See how half of the shell body came away intact, rather than breaking into small fragments? This means that the explosion was a lot less powerful and took a lot longer than a normal one. By contrast, take a look at how an artillery round is supposed to detonate:
These are different munitions, but still I hope you can appreciate the difference between the first “low order” explosion and the full “high order” one where the energy was released hundreds of times more quickly and with tiny, fast, and lethal fragments.
The obvious preference of everyone involved is to do the least destructive thing, but you always prepare for the worst to happen and, for that reason, you get people out of harm’s way. The only people who stay in harm’s way are the EOD team themselves. While they will obviously do everything in their power to carry out actions safely, the inherent uncertainty of the machines they’re dealing with mean that it can be deadly: at least 11 bomb technicians were killed in Germany since 2000.
…but you need to prepare for the worst
Once a bomb9 is discovered, the first thing that happens is a cordon and evacuation. The police cordon off an area a certain radius from the bomb and evacuate everyone within. The cordon can be as small as 100 m or as large as 1.5 km (see below). Obviously the bigger your cordon, and the more built-up the area, the more people you need to evacuated, and therefore the longer this stage will take.
While the evacuation is happening the EOD team can build some “protective works” around the bomb. These can consist of hundreds and hundreds of sandbags or dozens or much larger “Hesco” baskets, but in either case, the aim is to contain the blast and fragmentation effects of an explosion. You can see the aftermath of such protective works (including remnants of Hesco walls) in the photograph below taken after the detonation of a WW2 German bomb found in Kingston, near London, in 2019:
Nothing ever happens in EOD until the cordon and evacuation is complete. The size of the bomb, as well as the drills and procedures of the EOD team, will determine how wide the cordon is and therefore how many people need to be evacuated. A handful of recent examples from Germany and the UK illustrate this:
| Year | Location (link) | Size of bomb | Numbers evacuated | Cordon radius |
|---|---|---|---|---|
| 2016 | Augsburg (link) | 1,800 kg | 54,000 | 1,500 m |
| 2017 | Frankfurt (link) | 1,800 kg | 60,000 | 1,500 m |
| 2017 | Birmingham (link) | 250 kg | At least 80 (overnight) and major routes closed | 500 m |
| 2023 | Düsseldorf (link) | 1,000 kg | 13,000 | 500 m |
| 2024 | Plymouth (link) | 500 kg | 10,000 | 300 m |
As with everything public safety-related, police and EOD need to make a local assessment and balance the risk from the bomb against the risk caused by disrupting people for hours or days at a time. The cordon ensures that, whatever the EOD teams do, they can do it safe in the knowledge that their actions are extremely unlikely to kill or injure other people should things go wrong. This is why cordon and evacuation is the most important part of EOD.
Conclusion: The most disruptive aspect of EOD is also the most important
There’s a well-known story in EOD circles about the importance of cordon and evacuation. I couldn’t find open-source verification for this, so I’ll leave out the specific details10.
During an infamous terrorist bombing in a city centre in recent years, the well-practiced EOD team were able to deploy their robot to the suspect device before it exploded. They were about to take disruptive action which probably would have rendered it safe, but needed a cordon in place. Unfortunately a member of the press chose that moment to break the cordon and try to get a good picture of the robot doing its job. The police saw him and alerted the EOD team, who paused their operation while the police went back into harm’s way to forcibly remove the photographer. While hustling him away, the timer ran down and the IED detonated, injuring many and destroying large amounts of property.
The moral of the story is that evacuations are the only way to ensure you achieve the first and foremost pillar of EOD, which is the protection of life. Despite all the technical training and fancy equipment which EOD teams have, the most important part of our job is done by the beat cops. It starts before the team gets there and ends after they leave. And if it isn’t done right, then nothing else we do matters.
And that’s why my expat friend, on moving to Germany, had to learn that her new life now includes WW2-era bomb evacuations. Rare though they might be, they are very disruptive, but a necessity for the l EOD technicians who travel the other way, against the tide of escaping locals. These brave men and women face the uncertain threat of a deadly machine which was assembled long ago in a factory far away, then dropped from thousands of feet and buried by decades of concrete and soil. And they do it over and over again, in towns and cities across the country, and will be doing it for years to come.
That’s all we have time for this week. I realise this week’s post wasn’t very pop-culture heavy, but I don’t think there’s many films or TV shows out there which deal with EOD teams in their fight against very old munitions. Perhaps there’s an opportunity here? Anyway, thanks (as always) for reading. Please subscribe using the button below if you haven’t already, and please like and share this article with your friends. See you next time!
Featured Image: BBC: “Unexploded bombs: How common are they?” (2018).
PS: Sorry if the polls last week didn’t work well on your device. I’m still trying to figure out how to do these properly and hopefully will manage something slicker next time. Thanks to everyone who responded!
- Terminology/jargon diversion for those who are interested. What I’m talking about in this article is a subset of what we call “conventional munitions disposal” (CMD), which is in turn a subset of explosive ordnance disposal (EOD). The other main branch of EOD is improvised explosive device disposal (IEDD), which I’ve written about before (especially here and here) and which encompasses everything from small pipe bombs to sophisticated car bombs. The bombs I’m talking about today are technically called “air-dropped bombs”. Any unexploded item of ammunition is often referred to as “unexploded ordnance” (UXO) or “explosive remnants of war” (ERW).
Other types of UXO you might encounter in CMD include artillery projectiles, grenades, tank ammunition, mortars, rockets, and even guided weapons. ↩︎ - The weight of the bomb itself, not the weight of explosive inside it. These bombs had a charge-to-weight ratio of 30-40%. ↩︎
- Explosives are graded according to their sensitivity, which is their susceptibility to initiation from external shocks such as impact, friction, heat, etc. Some explosives are very insensitive (especially in newer munitions, where they are specifically designed as such), but even “classic” explosives like TNT are relatively insensitive, at least when compared to primary explosives or organic peroxide explosives like TATP. ↩︎
- There’s a whole science and practice around what kind of ammunition you can store with others and, as a result, what kind of safety distances you need between each storehouse. This is interesting for us because of recent goings-on in Russia, and I’m planning a whole article about this. ↩︎
- Of course this is a very basic example taking a very unlikely case. Most armourers or ammunition technicians will know how to screw in a fuze. Most flight crew will check. But the possibly of human error is there, however slight, and that’s the whole philosophy behind Murphy’s Law. With enough examples, it will happen. ↩︎
- “Submunitions” are the technical term for the many small bomblets which are dispersed from a single “carrier munition”. The complete system is known as a “cluster munition” or “cluster bomb.” ↩︎
- It’s generally a good idea to avoid kicking any fuzes. ↩︎
- As we discussed in the Hurt Locker post, the EOD Philosophy also includes “collect forensic evidence”, but this is less important in CMD and especially so in the case of legacy WW2 UXOs. ↩︎
- Or any UXO, or even any IED. ↩︎
- You may know which attack I’m talking about. If you can find open-source verification then please let me know and I’ll update. Otherwise let’s keep it anonymous. ↩︎
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