CHIEVRES, Belgium — In March 1918, a British intelligence unit set out to blow up a huge German ammunition dump in western Belgium.

By all accounts, it was an audacious mission, one that involved a Catholic priest who headed up a local spy network and the clandestine use of a German plane for the daylight insertion of a demolition expert.

“We would prefer to lose 10,000 men than to lose this munitions site,” Belgian air force Commandant Jan Savelkoels said, quoting a World War I German army general who was assigned to the region.

The mission succeeded, insofar as it effectively denied German forces use of that stockpile.

But the saboteurs failed to destroy all the munitions, something a Belgian explosive-ordnance disposal team is now addressing nearly nine decades later.

Savelkoels, the team commander, estimates that the site contains at least 300 tons of munitions, and that roughly 6 percent of it is toxic. The list of undesirable agents ranges from phosgene and diphosgene to chloramine, all of which were used by both sides in “the Great War.”

As far as Savelkoels knows, the site, near the U.S. air base at Chievres, is the biggest one of its kind from WWI. It alone will account for a normal year’s worth of recovered munitions.

In the interest of security, Belgian and U.S. officials asked that the exact location of the site not be disclosed.

“There are bombs I have never seen before,” said Savelkoels, a career EOD officer.

That says a lot, given that the Ypres region is still peppered with all sorts of ordnance, much of it dating to that era. Savelkoels said Belgian explosive-ordnance units annually get at least 3,000 requests.

“To see rounds that you studied about in (EOD) school is awesome,” said U.S. Army Master Sgt. Thomas Frankhouser, who recently visited the site and would be among those contacted in case of an emergency there. “We didn’t know it was here.”

For years, neither did members of the Belgian military, which lost or misplaced many of the documents pertaining to the ammunition site, Savelkoels said. Local residents brought the issue to the attention of Belgian authorities, but even then the details were sketchy.

The site was initially thought to cover about 130 square meters, Savelkoels said. But when his 12-man team began working the site in late April, they found it was more than four times that size. What was estimated to be a monthlong effort has turned into a six-month project.

“With every (passing) year, it is more and more dangerous,” Belgian army 1st Sgt. Dirk Gunst said.

After WWI, Belgian authorities began to work the site but lacked the expertise to handle it, so they buried it. A second effort commenced in the 1950s but it, too, was aborted.

“It’s an extraordinary site,” Frankhouser said.

The artillery rounds range in size from 7.7 cm to 25 cm, and the heaviest piece recovered so far checks in at 93 kilograms. Once the materiel is unearthed, it is either destroyed near the site, especially if it is deemed unstable due to leakage, or moved.

Every day, Savelkoels’ team checks the direction of the wind and other factors in the event an accident occurs and toxins get released into the air. So far, there have been no serious problems, though Savelkoels worries about complacency.

“Routine kills,” the Belgian officer said. “That’s what I fear here.”

That concern is negated somewhat by the extraordinary opportunity to get an extended view of a large cache of munitions nearly a century old.

“We’ve never found very large ammunition dumps from the second World War, only the first World War,” Savelkoels said, explaining the front lines were more static in World War I. “It’s the biggest one I’ve seen.”

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