In all, the war claimed the lives of 10 million soldiers and left 20 million more wounded, in addition to millions more civilian casualties.
It also left a huge number of explosives buried across Europe.
This demining team is based in Colmar in the Alsace region bordering Germany, an area France lost in the 1870s during the Franco-Prussian war then won back from Germany in 1918.
The team estimates that around 20 percent of the billion-and-a-half munitions fired during World War I did not explode.
And one hundred years later, Alsace remains particularly vulnerable, its residents living on a carpet of unexploded munitions that are still capable of death and destruction.
"World War I amounted to a laboratory for new munitions. The stalemate between opposing lines forced them to invent new artillery for the trenches. The munitions were conceived quickly, with improvised materials," said Schahl as he carefully stored the newly unearthed bomb in the back of a truck.
- Work for 'seven centuries' -
It's hard to estimate the number of munitions that still lie undiscovered in gardens, fields and Alsatian vineyards, but the 12-person demining team says their work is round the clock.
"Since the end of winter, we have received a dozen requests for intervention every day," said Schahl.
Every year, around 20 tonnes of shells, grenades and other mortars dating from the two world wars are collected by the unit. It is a titanic mission that will last another "seven centuries", he said.
Disposing of them is a delicate process. The munitions are carefully removed and transported to remote gravel pits.
The locations are secret to keep the public out of harm's way. They are then attached to anti-tank mines and blown up.
Another call has just come in. On the way to the picturesque commune of Aspach-le-Haut, the employees of a waste disposal company have just unearthed some chemical weapons shells, scattered in a pile of rocks during a crushing operation.
It was a miracle that no explosion took place, they say.
"In cases like this, there are huge constraints on how we explode these munitions," said one of the demining team, Frederic, who is scratching the warheads of the munitions with gloved hands to determine their composition.
They may look dilapidated, but these bombs have lost nothing of their dangerous potential. "With oxidation, the explosive degrades and becomes more unstable than before," said Frederic.
In Alsace, rare catastrophes have left an indelible mark on the community. In 1981, six children were killed when they moved a mortar in their schoolyard in Bremmelbach.
"That was the start of the population taking notice," recalls Edouard Hannauer, one of the deminers from that time.
But popular assistance is not always a good idea.
"People would bring us munitions they had found," said Hannauer, even though the vast majority of accidents occur when people try to move munitions themselves.
Even the experts are prone to grave accidents. In 2007, two deminers died in Metz while preparing munitions for explosion.
At one of the secret gravel pits, Schahl is about to press the red button on his detonator.
"Four, three, two, one, fire!" he shouts, triggering a huge explosion that puts one more potential death trap out of harm's way.