Military explosive ordnance disposal experts examine an unexploded 500-pound World War II bomb discovered at a building site in Bethnal Green, London, on Aug. 10. (Sgt. Ross Tilly/Ministry of Defense via Reuters)

The knock came after residents of the east London apartment complex had already gone to bed. They opened their doors to find someone in uniform standing before them: a police officer, a firefighter, a member of the army.

A 500-pound bomb had been found a few hundred feet away, the officers said. They needed to get out.

It’s an alarming message for anyone to receive while standing in their pajamas, but especially so for someone in a city that hasn’t seen active conflict in seven decades. How did an explosive wind up beneath this leafy London neighborhood?

It was probably dropped by a German bomber during the blitz of British cities at the beginning of World War II. Unlike most of the other bombs dropped during the attack, this one didn’t explode and instead sunk deep into the London clay, where it lay dormant for 70 years.

Working through the night, disposal experts were finally able to defuse the device Tuesday morning, and residents were allowed to return home.

It wasn’t the first time and almost certainly not the last that people will be pulled from their beds over a relic of a war fought long ago. Despite the distance of years and international protocols aimed at preventing them, still-active bombs — called unexploded ordnance — linger underground for years after conflicts are over.


Some of the many bombs discovered years later. (AP, AFP, Getty Images)

[East London exhales after unexploded World War II bomb is defused]

This was the third device the British Army has had to detonate in London this year. But the problem isn’t only in the United Kingdom, nor is it only from WWII.

The oldest unexploded ordnance still being discovered date back to the Civil War, the first conflict to be characterized by the use of explosive shells.

In 2008, Virginian Sam White — a Civil War history buff who crisscrossed the state looking for relics of past battles — was attempting to restore a 1860s cannonball when it exploded, killing him and sending a chunk of shrapnel through the front porch of a home a quarter of a mile away.

The fatal blast was unusual for a Civil War relic — especially in the hands of someone like White, who had previously disarmed an estimated 1,600 shells for collectors and museums, according to the Associated Press. Explosives aren’t often still potent a century and a half after they were made.

But the Union and Confederate troops lobbed an estimated 1.5 million artillery shells and cannonballs at one another during the war, according to the AP, and as many as 20 percent were duds (explosives that failed to detonate on time). Some inevitably end up still posing a threat.

Soldiers from 20th CBRNE Command (Chemical, Biological, Radiological, Nuclear, Explosives) destroyed a live Civil War-era artillery round discovered in Ilwaco, Wash., April 22. The artillery round was an Absterdam Type 2/3 projectile. U.S. Army bomb squad Soldiers disposed of a Civil War cannon ball that was found in a Georgetown townhouse's chimney Jan. 21. 2015
Left:  A live Civil War-era artillery round was discovered and then destroyed in Ilwaco, Wash., in April. Right: U.S. Army bomb squad soldiers disposed of a Civil War cannonball that was found in a Georgetown townhouse’s chimney in Washington, D.C. (Photos courtesy of CBRNE)

In the small farming towns of France and Belgium, undetonated World War I explosives that turn up during each year’s spring planting and autumn plowing are known as the “iron harvest.”

More than a billion shells were fired during the conflict, according to the BBC, and as many as a third never exploded. In 1996, the French Interior Ministry estimated that 12 million shells still slumber in the soil near Verdun alone. So many explosives linger from century-old battles that residents often see their discovery as utterly banal.


Every year, farmers unearth several tons of shells, shrapnel, gas shells and unexploded grenades that bomb-disposal experts of Amiens remove and destroy. Top: A French farmer plows his field outside Courcelette, the scene of a WWI battlefield in the Somme in 2014. Bottom: Unexploded shells wait for their removal by bomb-disposal experts after a farmer found them in Colincamps, France, in 2014. (Pascal Rossignol/Reuters)

In 2007, NPR reporter Eleanor Beardsley described farmer Xavier Vandendriesche, from the small village of Courcelette, discovering a grenade from the Battle of the Somme in his potato field.

“Vandendriesche has found a grenade. It’s round, brown and covered in dirt. It looks just like a potato,” she said.

He gave it to a Michel Colling, a de-mining expert, who buried it with other uncovered explosives attached to a long electric cable.

“He then moves about 75 yards away and hits the detonator button,” Beardsley narrated, while Colling counted in French: “Une, deux, trois!”

There was a muffled pop, and Beardsley saw smoke steam from the ground. The shells had been rendered harmless.

It’s a lot of work, Colling said, but necessary.

“These shells are charged and still active,” he told Beardsley. “We can’t forget that these are weapons that were made to kill people — and they still do.”

Since 1946, when France’s Département du Déminage (Department of Mine Clearance) was established, more than 630 démineurs (deminers) have been killed in the line of duty, according to Donovan Webster’s book “Aftermath: The Remnants of War.” So have an uncounted number of civilians.

The démineurs Webster spoke to said that the most-feared unexploded ordnance were the toxic ones, shells filled with now-banned poisons like mustard gas.

“You never know how solid their skins are. They are often very rusty, so they may leak gas and kill you as they lift them,” Henry Bélot said. Bélot was later seriously injured when poison leaked from a shell and into his gas mask.

“Every day you can die. It’s something you remember each morning,” he told Webster. “You can’t anticipate it. Out there is a shell with your name on it. Today, if you lift it, you are in the past.”

After WWII, international agreements began to require warring countries to deal with unexploded ordnance left over from battles. But the efforts weren’t always effective. Neighborhoods in London, Portsmouth, Berlin, Munich, Tokyo, Okinawa and other cities are periodically evacuated while officials deal with abandoned explosives.


A French soldier from the 2nd Engineer Regiment of the French Foreign Legion carries an explosive device out of the village of Dasawe Aole, north of Kabul, Afghanistan, on Aug. 6, 2002. (Natalie Behring-Chisholm/Getty Images)

According to a 2008 article in Der Spiegel, discoveries of unexploded ordnance are a weekly occurrence in Germany. The effort to uncover them is often a race against time: buried bombs become more and dangerous as their detonators are eroded by exposure.

“In the last few years we’ve found that the detonators we take out of such bombs are increasingly brittle,” bomb disposal expert Hans-Jürgen Weise told the German newspaper. “Recently we’ve had three extracted detonators go off with a pissssh sound while they were being transported away, all it took was a bit of vibration. One day such bombs will be so sensitive that no one will be able to handle them.”

International law requires that countries clear areas under their control of the “explosive remnants of war” after the end of hostilities. But according to the Mines Advisory Group, nine people a day are killed by landmines and unexploded ordnance worldwide. They are the lethal legacy of conflicts in places like Lebanon, Vietnam, Angola and others.

But nowhere is the threat of unexploded ordnance more pervasive than Laos, which has the distinction of being the most heavily bombed country per capita in history.

According to the advocacy organization Legacies of War, more than 2 million tons of ordnance were dropped on Laos by the United States during the Vietnam War — the equivalent of a planeload of bombs every eight minutes, every day for nine years. Up to a third of those bombs didn’t detonate, Legacies of War says

In the four decades since, more than 8,000 people have been killed and 12,000 wounded by leftover live explosives, the New York Times reported in April. Much of the harm is caused by cluster bombs, which spew dozens of smaller bomblets when detonated and can rip apart an area as large as a football field.

“This country, every time I’ve been here, blows my mind,” Tim Lardner, a bomb disposal expert who has worked in Laos and other countries for 25 years, told the Times. “The scale of the contamination is horrendous.”


Clockwise from top left: Two Laotian boys play with their ball in a front yard near collected bomb canisters. (AP) Xay, 19, recovers in a hospital in Phongsavan on May 4, 1996, after being burned by a U.S.-made white phosphorus shell he and a friend opened near their village in northern Laos. (Denis Gray/AP) Laotian women walk past bombs and shells that lie next to the streets in this northern province. (Reuters) Stuart Broome, technical field manager for the U.K.-based Mines Advisory Group (MAG) and supervisor of Laotian bomb-clearance teams in the province, in 2004. (Kathrine Ratcliffe/AFP/Getty Images)

The tennis-ball-size explosives, known as “bombies,” are often triggered by farmers or playful children. The Christian Science Monitor spoke with a woman whose 9-year-old brother was killed when he struck a bomb with a hoe. The Times told the story of two children who tossed one around, thinking it was a pétanque ball (used for a French game similar to bocce). When the “ball” burst, one child was killed instantly. Another, along with a mother who had been standing nearby, died of their injuries days later. Two others were left limping by the blast, their injuries untreated a year later.

“People’s familiarity is the most striking thing for me,” Jo Pereira, an occupational therapist with the Lao charity Cope, which fits victims of unexploded ordnance with prosthetic limbs, told the Guardian. “They’ve lived with it for so long. Much of it is in their houses. Children think ‘we’ve got those at home’ and don’t see the risks.”

Advocacy efforts have persuaded the United States to more than quadruple its spending on bomb removal in Laos — according to the Times, the federal government dedicates $12 million annually to dealing with unexploded ordnance there, up from $2.5 million a decade ago.

Still, only 1 percent of the bombs have been cleared, according to Legacies of War.

“We keep on digging,” Houmphanh Chanthavong, a Laotian government official working on bomb education, told the Times. “And we keep on finding more.”